Add rbenv support

* Add .rbenv-gemsets file for isolating the environment (gems)
* Add .gems folder to .gitignore

@thanpolas this is COMPLETELY OPTIONAL : 

* If someone manages ruby versions and gem environments with [rbenv](https://github.com/sstephenson/rbenv) and its plugin [rbenv-gemset](https://github.com/jf/rbenv-gemset) then this is essential.
* If not then no harm done (they will be ignored)

You can merge it (I would)...or not. It's your call :-)