test: cover supported node versions with travis

Author: rvagg

.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: