Add require of mattr_accessor since Compatibility relies on it.

Follow up to
https://github.com/rails/rails/commit/c9c5788a527b70d7f983e2b4b47e3afd863d9f48

activesupport/lib/active_support/core_ext/date_and_time/compatibility.rb

+require 'active_support/core_ext/module/attribute_accessors'
+
 module DateAndTime
   module Compatibility
     # If true, +to_time+ preserves the timezone offset of receiver.