use base_class method for STI compatibility

lib/acts_as_follower/follower_lib.rb

@@ -8,7 +8,7 @@ module FollowerLib
     # Retrieves the parent class name if using STI.
     def parent_class_name(obj)
       unless parent_classes.include?(obj.class.superclass)
-        return obj.class.superclass.name
+        return obj.class.base_class.name
       end
       obj.class.name
     end