Remove ruby-head from Travis build

ruby-head has a known issue with Travis that causes build errors due to
an issue installing bundler. It's not worth testing against.
Add Ruby 2.0 to Travis build
Add Rubinius to Travis build
Remove travis message
Merge pull request #57 from carlosdp/remove-ruby-head

Remove ruby-head from Travis build and add Rubinius/Ruby 2.0