Enhance Sprig-Reap's Abilities to Handle Polymorphism

Sprig-Reap was unable to properly identify dependencies and polymorphic-associated classes.  This PR adds some additional intelligence around that logic.

Eli and I paired on this stuff.  We also cleaned up small things here and there.
This and the later stub seem a bit tricky. Could you actually create some classes so this doesn't act on a pretty heavily set up mock? 

```
before do
  class MyModel < ActiveRecord::Base; end
  class AnotherModel < ActiveRecord::Base; end
end

its(:polymorphic_dependencies) { should == [MyModel, AnotherModel] }
```
?