Raise if a method is defined with a FactoryGirl block (factory or trait)

This resolves an issue where users have defined methods inside factory
girl blocks. This hasn't worked in quite a while and without raising,
users would be able to define methods and see an odd error (a
NoMethodError for `singleton_method_added=`). Instead, we raise
immediately so that no issues come up later when they actually try to
use the factories.

Closes #513