Update Rubinius Gemfiles, Travis

Simplify the Gemfile setup for Travis by specifying `rbx-2`.

This commit updates the Travis version of Rubinius to 2.2.5+, which includes the ruby standard library. Thus platform-specific Gemfile configuration is no longer needed for Rubinius.

Also, Rubinius 2.2.5 does not work with sqilte3 1.3.9, so require sqlite3 version 1.3.8 until it does.

:computer: To run the tests locally, you will need to use Rubinius 2.2.5+.

See http://www.benjaminfleischer.com/2013/12/05/testing-rubinius-on-travis-ci/