chore(travis): use version 'stable' of nodejs

Author: Alexandre-io

.travis.yml

@@ -1,10 +1,6 @@
 language: node_js
 node_js:
-  - '5'
-  - '4'
-  - '3'
-  - '2'
-  - '0.12'
+  - 'stable'
 before_script:
   - npm install grunt-cli -g
 script: npm install . && npm test