Use Node idioms for Travis
Use Node idioms for Travis

- Sets language from `objective-c` to `node_js`.
- Removes `os` entries, because it should work the same on Linux or OSX.
- Uses `node_js` matrix key to specify versions.
- Uses `npm` cache strategy.
- Removes redundant steps under `script` that installed the latest version of npm (because those steps had been failing).
Mozu/mozu-node-sdk