stub on the version-appropriate association class

refs CNVS-9166

rails2.3 calls it AssociationProxy, rails3 just calls it Association.

Change-Id: I28d52d0a50a7f8d3269a92c64676c171c11a9239
Reviewed-on: https://gerrit.instructure.com/26233
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>

lib/stubs/shard.rb

@@ -117,7 +117,7 @@ ActiveRecord::Base.class_eval do
 end
 
 module ActiveRecord::Associations
-  AssociationProxy.class_eval do
+  (CANVAS_RAILS2 ? AssociationProxy : Association).class_eval do
     def shard
       Shard.default
     end