BF/ZPH Extract directories from rake task, fix rake test, ensure we are testing against the rails fork

@bf4 @zph 

* Ensure we are testing against the rails fork, so that when the dummy app loads rails, it doesn't have a `rake stats` task, but has a `rake code_metrics:stats` task
* fix rake test to test against `rake code_metrics:stats` not `rake stats`
* Extract directories from rake task in a way that is compatible with usage of the code statistics in the wild (at least if the class names are changed). [1](https://github.com/rspec/rspec-rails/blob/d546278af1/lib/rspec/rails/tasks/rspec.rake#L41) [2](https://github.com/cavalle/steak/blob/284be93089/lib/tasks/steak_tasks.rake#L9) [3](http://stackoverflow.com/questions/1613655/rake-stats-and-cucumber) [4](https://github.com/jnicklas/turnip/issues/45)  [5](https://github.com/nsi-iff/bd/blob/999b2057df031d8ec90e6f3ee450b861aaa5f834/lib/tasks/rspec.rake)