test: cover supported node versions with travis

PR-URL: #1809
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: cclauss <cclauss@me.com>

.travis.yml

@@ -5,8 +5,6 @@ matrix:
   include:
     - name: "Python 2.7 on Linux"
       python: 2.7
-    - name: "Python 3.7 on Linux"
-      python: 3.7
     - name: "Python 2.7 on macOS"
       os: osx
       osx_image: xcode10.2
@@ -18,6 +16,18 @@ matrix:
       node_js: 12  # node
       env: PATH=/c/Python27:/c/Python27/Scripts:$PATH
       before_install: choco install python2
+    - name: "Node.js 6 & Python 3.7 on Linux"
+      python: 3.7
+      before_install: nvm install 6
+    - name: "Node.js 8 & Python 3.7 on Linux"
+      python: 3.7
+      before_install: nvm install 8
+    - name: "Node.js 10 & Python 3.7 on Linux"
+      python: 3.7
+      before_install: nvm install 10
+    - name: "Node.js 12 & Python 3.7 on Linux"
+      python: 3.7
+      before_install: nvm install 12
   allow_failures:
     - os: windows
 install:
@@ -26,7 +36,7 @@ install:
 before_script:
   - flake8 --version
   # stop the build if there are Python syntax errors or undefined names
-  - flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics
+  - flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
   # exit-zero treats all errors as warnings.  Two space indentation is OK.  The GitHub editor is 127 chars wide
   - flake8 . --count --exit-zero --ignore=E111,E114,W503 --max-complexity=10 --max-line-length=127 --statistics
   - npm install