update active_polymorph for rails 4.0

fixes CNVS-15774

test plan:
 * take a quiz. it shouldn't asplode

Change-Id: I090fc549cd5d7986d6963fb4ae6be441eda2c675
Reviewed-on: https://gerrit.instructure.com/41659
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>