Fixes has_many conditions in Rails 4.1

Rails 4.1 passes the association into the scope lambda, not the class.
Rather, the associated object's class is available within the context
of the lambda as `model`.

lib/paper_trail/has_paper_trail.rb

@@ -61,7 +61,7 @@ module PaperTrail
 
         if ::ActiveRecord::VERSION::MAJOR >= 4 # `has_many` syntax for specifying order uses a lambda in Rails 4
           has_many self.versions_association_name,
-            lambda { |model| order(model.version_class_name.constantize.timestamp_sort_order) },
+            lambda { order(model.timestamp_sort_order) },
             :class_name => self.version_class_name, :as => :item
         else
           has_many self.versions_association_name,