Adds travis: Ruby 2.0.0 temporally remove ruby-head

Waiting to fix the error below to add ruby-head again.

> Bundler::GemNotFound: Could not find json-1.8.1.gem for installation
> An error occurred while installing json (1.8.1), and Bundler cannot
> continue.
> Make sure that `gem install json -v '1.8.1'` succeeds before bundling.