Use ActiveSupport delegation over Ruby Forwardable

The delegation style found in the AssociationMatcher
is an ActiveSupport style; requiring `forwardable`
without extending it does not add delegation support.

lib/shoulda/matchers/active_record/association_matcher.rb

@@ -1,4 +1,4 @@
-require 'forwardable'
+require 'active_support/core_ext/module/delegation'
 
 module Shoulda # :nodoc:
   module Matchers