Fix da build

This can probably be improved but [Travis is happy now](https://travis-ci.org/Helabs/pah/builds/25910471) :fireworks: 

So.... The root cause is that when running the commands from within aruba / cucumber, the `BUNDLE_GEMFILE` env var is set, tricking the interactions with bundler that happen within the application templates. Wrapping them with `Bundler.with_clean_env` fixes the issue.

I've also done some other tweaks as an attempt to make the build run faster. Let me know what you think about that ;-)

/cc @TiuTalk @maurogeorge