Add NodeJS v5.x & v7.x to Travis CI configuration

Author: christophervoigt

packages/stylelint-config-airbnb/.travis.yml

@@ -6,7 +6,9 @@ cache:
     - node_modules
 node_js:
   - "8"
+  - "7"
   - "6"
+  - "5"
   - "4"
 before_install:
   - npm prune