Drastically reduce the number of builds that are made with Travis

Based on research in https://github.com/npr/pmp-php-sdk/issues/36,
Travis CI runs tests using the following PHPUnit versions:

- PHP 5.6: PHPUnit 5.7.23
- PHP 7: PHPUnit 6.4.3
- PHP 7.1: PHPUnit 6.4.3
- PHP 7.2: PHPUnit 7.0.2

This commit reduces the number of tests that are run by more than
two thirds. Before, we were running 4 different PHP versions and
11 diffferent WordPress versions. Many of those combinations were
incompatible, for different reasons:

1. There are PHP warnings about deprecated PHP-4-style
   constructors.
2. Requires PHPUnit 5 because PHPUnit 6 support was not
   included until WordPress 4.7.
3. Unless there's been an update since
   [this trac thread](https://core.trac.wordpress.org/ticket/39822),
   WordPress tests still assume PHPUnit 6, and Travis runs PHP 7.2
   tests with PHPUnit 7

| phpunit | 5.7 | 6.4 | 6.4 | 7.0 |                 |                  |
| php     | 5.6 | 7.0 | 7.1 | 7.2 |                 |                  |
| WP      |     |     |     |     | wp release year | then-current php |
|---------|-----|-----|-----|-----|-----------------|------------------|
| 3.8     |     | 1   | 1   | 1   | 2013            |                  |
| 3.9     |     | 1   | 1   | 1   | 2014            | 5.5              |
| 4.0     |     | 1   | 1   | 1   | 2014            | 5.5              |
| 4.1     |     | 1   | 1   | 1   | 2014            | 5.5              |
| 4.2     |     | 2   | 2   | 2   | 2015            | 5.6              |
| 4.3     |     | 2   | 2   | 2   | 2015            | 5.6              |
| 4.4     |     | 2   | 2   | 2   | 2015            | 7.0              |
| 4.5     |     | 2   | 2   | 2   | 2016            | 7.0              |
| 4.6     |     | 2   | 2   | 2   | 2016            | 7.0              |
| 4.7     |     |     |     | 3   | 2016            | 7.1              |
| 4.8     |     |     |     | 3   | 2017            | 7.1              |
| 4.9     |     |     |     | 3   | 2017            | 7.2              |

Accordingly, this commit removes PHP 7.2 from tests, and replaces the
separate lists of PHP and WordPress versions with one list of selected
PHP and WordPress versions.

.travis.yml

@@ -8,26 +8,65 @@ sudo: false
 language: php
 
 # PHP versions
-php:
-    - "5.6"
-    - "7.0"
-    - "7.1"
-    - "7.2"
+# php:
+#     - "5.6"
+#     - "7.0"
+#     - "7.1"
+#     - "7.2"
 
 # WordPress versions
-env:
-    - WP_VERSION=3.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=3.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.0 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.1 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.2 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.3 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.4 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.5 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.6 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.7 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
-    - WP_VERSION=4.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+# env:
+#     - WP_VERSION=3.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.0 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.1 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.2 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.3 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.4 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.5 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.6 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.7 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+#     - WP_VERSION=4.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+
+# for a breakdown of why these versions were chosen, see https://github.com/INN/WP-DS-NPR-API/issues/12#issuecomment-374730094
+matrix:
+    include:
+      - php: 5.6
+        env: WP_VERSION=3.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=3.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.0 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.1 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.2 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.3 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.4 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.5 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.6 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.7 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 5.6
+        env: WP_VERSION=4.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 7.0
+        env: WP_VERSION=4.7 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 7.0
+        env: WP_VERSION=4.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 7.0
+        env: WP_VERSION=4.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 7.1
+        env: WP_VERSION=4.7 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 7.1
+        env: WP_VERSION=4.8 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
+      - php: 7.1
+        env: WP_VERSION=4.9 WP_TESTS_DIR=/tmp/wordpress/tests/phpunit WP_CORE_DIR=/tmp/wordpress
 
 branches:
     only: