.travis.yml: changing node version specification

using explicit number for LTS and for archaic ones
using 'node' for latest stable (add specific one when it becomes LTS)

.travis.yml

@@ -2,8 +2,8 @@ language: node_js
 node_js:
   - "0.10"
   - "0.12"
-  - "4"
-  - "5"
+  - "4.2"
+  - "node"
 install:
   - npm install -g grunt-cli mocha
   - npm install