Specify Rubinius 2.x for Travis.

This will ensure that as newer versions of Rubinius 2.x are released,
this project will be tested on them. Older versions of 2.x shouldn't
need to be supported as most users will be updating with new releases.

See more http://rubini.us/2013/12/03/testing-with-rbx-on-travis/
Add gems for Rubinius.