Test against Rubinius 2
Test against JRuby in additional modes
Test against Ruby 2.1.0
Test against ruby-head
Add temporary workaround for failing build on Ruby 1.8.7

Rubygems broke builds on Ruby 1.8.7:

https://github.com/travis-ci/travis-ci/issues/1793
https://github.com/rubygems/rubygems/pull/763

This patch is a temporary workaround until rubygems is updated.