Fix error calling private method in rails 4.0

CHANGELOG.md

@@ -22,6 +22,8 @@ recommendations of [keepachangelog.com](http://keepachangelog.com/).
 
 - [#925](https://github.com/airblade/paper_trail/pull/925) - Update RSpec
   matchers to work with custom version association names
+- [#929](https://github.com/airblade/paper_trail/pull/929) -
+  Fix error calling private method in rails 4.0
 
 ## 6.0.2 (2016-12-13)
 

lib/paper_trail/record_trail.rb

@@ -474,7 +474,9 @@ module PaperTrail
       if @in_after_callback && RAILS_GTE_5_1
         @record.attribute_before_last_save(attr_name.to_s)
       else
-        @record.attribute_was(attr_name.to_s)
+        # TODO: after dropping support for rails 4.0, remove send, because
+        # attribute_was is no longer private.
+        @record.send(:attribute_was, attr_name.to_s)
       end
     end