"fix" CollectionAssociation#uniq in rails 3

Change-Id: I2c81185934cf588cd809ab4d08e4d6408f642425
Reviewed-on: https://gerrit.instructure.com/29570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>