Switch to using recommended Node.js version on Travis

no issue
- developers should always be using our recommended Node.js version
- having Travis run on an older Node.js version was causing issues occasionally because dependencies would inadvertently drop 4.x support, this is mostly a non-issue for Ghost-Admin because the supported node version is only relevant to developers and build environments

.travis.yml

@@ -3,7 +3,7 @@ dist: trusty
 
 language: node_js
 node_js:
-  - "4"
+  - "6"
 
 env:
   global: