Fix issue with Gemfile not having the right ruby version

Since we removed the dependency with the Heroku gem, a lot of functionalities stopped working due to a mismatch in the Bundler Ruby version and the current dir.

`[...]/bundler/definition.rb:361:in 'validate_ruby!': Your Ruby version is 1.9.3, but your Gemfile specified 2.0.0 (Bundler::RubyVersionMismatch)`

Shelling out the commands we run fix this issue as described in the Bundler documentation (ref: http://bundler.io/v1.3/man/bundle-exec.1.html)

This should also fix Issue #15