fix grade_summary_presenter for rails 3

Change-Id: Ie3cbe5a8b89aec8b2d5fa6eb889aea4971bd1299
Reviewed-on: https://gerrit.instructure.com/31008
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>