implement specs for run_on_modifications and make specs pass

This pull request contains the following:

* Implemented specs for `Teaspoon#run_on_modifications`.
* If i'm not mistaken, teaspoons `Console#execute_without_handling` returns true if the specs pass and false if they don't. This behaviour wasn't correctly replicated in guard-teaspoon.
* Updated Gemfile to use the same gems as teaspoon does, in order to make the specs pass that use `bundle install --local`