fix model cache for rails 3

Change-Id: I642731a7bb26580bf236fd275fd8ce30565ef2eb
Reviewed-on: https://gerrit.instructure.com/29553
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>