Add support for Mongoid

* Code adapted from https://github.com/bowsersenior/workflow_on_mongoid
* Update adapter activation code to handle the case when more than one ORM class is defined
* Add tests to ensure everything works with Mongoid adapter
* Update gemspec to include mongoid
* Update .travis.yml to add mongodb service for mongoid tests
* Tweak gemspec and gemfiles/Gemfile.rails-*
* ignore .lock files in gemfiles folder