explicitly save quiz submissions for rails 3 compatibility

touch no longer runs save callbacks

Change-Id: I1a0e703cfc196c77cba91d233f10a1e8ee138196
Reviewed-on: https://gerrit.instructure.com/29575
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>