From http://docs.travis-ci.com/user/languages/ruby/

In reference to #102

"If you don't specify a version, Travis CI will use MRI 1.9.3 as the default.

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."