fix assignment_rubrics_spec for rails 3

Change-Id: I3e2365031b4be77b43ceda35a960c88c4f3b0723
Reviewed-on: https://gerrit.instructure.com/31811
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>