fix gradebooks_controller_spec for rails 3

Change-Id: I927859cd69aecf78c593cc5507144a01fc7340ed
Reviewed-on: https://gerrit.instructure.com/30094
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>