Update Rubinius version in .travis.yml

Travis now supports only Rubinius 2.1 and
up. http://about.travis-ci.org/docs/user/languages/ruby/#Rubinius

Additionally the Ruby standard library has been separated from rbx in to
a set of gems and now needs to be set as a development dependency.
http://rubini.us/2013/09/22/ready-set/