Chore/update getting started

Highlights:
* Add `Prerequisites` section to `GETTING_STARTED.md` stating that we're using `Java 1.7`.
* Update `.gitignore` 
  * add  `.java-version` (generated by [jEnv](http://www.jenv.be/)) 
  * add `.ruby-gemset` (generated by [RVM](https://rvm.io/))