Cache bundle results on Travis to S3

Based on [Mislav post](http://mislav.uniqpath.com/2013/12/travis-cached-bundle/) on speeding up the performance of `hub` test suite. I tweaked the scripts to support the multiple Gemfile shenanigans a bit but it seems to be working fine on my fork. We can compare the duration on https://travis-ci.org/lucasmazza/devise and https://travis-ci.org/plataformatec/devise.

The only downsize of this implementation is that we need to keep the `.lock` files in the repository so the script can download the bundled gems for the current lockfile. I know that its a pain in the ass, but we could get some rake tasks to deal with updating these files or something like that - or even try to reduce the number of Gemfiles in the project.

Also, I need to ping @georgeguimaraes before pushing this one upstream.