Speed up dependency resolution on Travis.

[hermannloose]

Most of the time on Travis is spent installing dependencies, which could probably be reduced following this, thus reducing load on Travis.

[hermannloose]

Also, start using the twiddle wakka.

[hermannloose]

Seeing that a lot of articles suggest that a fresh checkout with Gemfile and Gemfile.lock being in sync should result in Gemfile.lock having authority on which versions are installed, there might not actually be any room for improvements on Travis in this regard. Keeping this open until I have a more thorough understanding of what is happening.