the rbx version must be specified. Addresses #12

Not sure if there are other version of Rubinius that you would like to add to the travis build, but here is the fix from the documentation.

    When using Rubinius, there's currently an issue with selecting the correct version in RVM
     in our build environment, but only when specifying rbx as your version. 
    As a workaround, specify rbx-2 instead.

Found [here](http://docs.travis-ci.com/user/languages/ruby/).