Run the Travis build against the last 2.x Rubinius

Hello,

This is just a very tiny pull request to make our build green.

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.

Have a nice day.