Specify TravisCI to run rbx-2 per @brixen

see http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-ci/ https://github.com/tarcieri/http/pull/79#issuecomment-34628465

The ‘rbx-2’ is to limit testing on Rubinius 2.x since there will be other versions of Rubinius > 2.x