Support gemfile key on source and matrix

With this PR we build our matrix taking into account the gemfile key on the travis.yml files.

Besides I added a Gemfile as the default Gemfile, that was the previous behaviour.

I'm not sure I followed your code/spec conventions If not just let me know.

One thing I notice is that I wasn't able to test that the middleware are building the proper script using the gemfile keys, how can I add that?