.travis.yml: Add newer Node.js versions

Closes mathiasbynens#21.

Author: darrachequesne

.travis.yml

@@ -2,7 +2,9 @@ language: node_js
 node_js:
   - "0.10"
   - "0.12"
-  - "iojs"
+  - "4"
+  - "5"
+  - "6"
 before_script:
   - "npm install -g grunt-cli"
   # Narwhal uses a hardcoded path to openjdk v6, so use that version