pass proxy_association to scopes in rails 3

Change-Id: I6f0b7bfc5ab9d9c73287e8b51f522406133a9091
Reviewed-on: https://gerrit.instructure.com/30041
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>