Add filename to whodunnit warning

lib/paper_trail/frameworks/rails/controller.rb

@@ -95,7 +95,11 @@ module PaperTrail
         user_present = user_for_paper_trail.present?
         whodunnit_blank = ::PaperTrail.whodunnit.blank?
         if user_present && whodunnit_blank && !@set_paper_trail_whodunnit_called
+          source_file_location = self.class.instance_methods(false).map { |m|
+            self.class.instance_method(m).source_location.first
+          }.uniq.first
           ::Kernel.warn <<-EOS.strip_heredoc
+            #{source_file_location}:
             user_for_paper_trail is present, but whodunnit has not been set.
             PaperTrail no longer adds the set_paper_trail_whodunnit callback for
             you. To continue recording whodunnit, please add this before_action