Run the Travis build against the last 2.x Rubinius

Since Rubinius versions are changing very oftenly, let's run the build
on the 2.x version to have a certain degree of locking and not being
obliged to update the Travis configuration time on each release.

Also see http://rubini.us/2013/12/03/testing-with-rbx-on-travis/ for
more information.
Add the Ruby Standard Library and Racc gems

If we are bundling on Rubinius, let's install the rubysl and racc gems
to get a green build on Travis
Merge pull request #328 from vmg/rubinius

Run the Travis build against the last 2.x Rubinius