Allow to fail on ruby-head
Update Rubinius Gemfiles, Travis

Simplify the Gemfile setup for Travis by requiring version 2.2.5.

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.
Merge pull request #522 from teeparham/rubinius_2

Update Rubinius Gemfiles, Travis