Update Travis tests for Ubuntu Precise.

[benlk]

Basically, copy the work from npr/nprapi-wordpress#46 and put it in this plugin's .travis.yml, and make sure that the tests run. They don't have to run successfully for this issue to be marked complete, but they do need to run.

[benlk]

Copying from npr/nprapi-wordpress#46:

Based on research in npr/pmp-php-sdk#36,
Travis CI runs tests using the following PHPUnit versions: (unknown where exactly these numbers came from; I can't find exact deets)

• 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

Reasons that given versions of WordPress have tests incompatible with travis-ci.org:

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, WordPress tests still assume PHPUnit 6, and Travis runs PHP 7.2 tests using 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		2	2	2	2016	7.1
4.8				3	2017	7.1
4.9				3	2017	7.2
5.0				3	2018	7.3
5.1				3	2019	7.3

During conversation today, we decided to target the last pre-Gutenberg WordPress version and the most-recent 2 releases: 4.9, 5.0, 5.1.

For PHP versions, I think we should take the same approach: current supported and last pre-7 PHP version.

[benlk]

PHPunit 8 requires PHP > 7.2, and the WordPress trac ticket for PHPunit 8 support is this one.

The list of currently-supported PHPunit/PHP combinations can be found at https://phpunit.de/supported-versions.html

The list of currently-supported PHP versions can be found at https://secure.php.net/supported-versions.php

[benlk]

A review of tests initially introduced in #138: https://travis-ci.org/INN/link-roundups/builds/502881348?utm_source=github_status&utm_medium=notification

• Travis is running PHP 7.2 with PHPunit 8. https://travis-ci.org/INN/link-roundups/jobs/502881355
• Travis is running PHP 7.3 with PHPunit 8. https://travis-ci.org/INN/link-roundups/jobs/502881358
• WP 4.9/5.0 don't have PHPUnit 8 forwards-compatibility shims in place; 5.1 does provide a warning about 8 not being supported.