Improve Travis' feedback

Hello,

Add 2.1.0 to the list of tested rubies but also keep ruby-head to be sure to be compatible with the current trunk since Ruby moved to semantic versioning, the release process is a bit faster.

Also simply test on the latest Rubinius version and allow failures on it until we can find an appropriate fix for the code introduced in #333.

We should also rely on the travis_retry command when running bundle install since the rake-compiler gem set up often generate a Bundler error.

Have a nice day.