fix simply versioned association for rails 3

Change-Id: I9545f224ef8340f3efeda04682999ace811c2352
Reviewed-on: https://gerrit.instructure.com/28535
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>