Update .travis.yml

Specifying 'rbx-2' will use the newest Rubinius 2.x that the Travis RVM can install. The current Rubinius stable release is 2.2.9. AFAIK, the Travis RVM is still installing 2.2.6 for 'rbx-2', but that is at least a lot newer than 2.1.1.
Merge pull request #15 from brixen/patch-1

Update .travis.yml