fix submissions_api_controller for_students for rails 3

Change-Id: I939085f973ed93b13f28552c31179145fd3f2c27
Reviewed-on: https://gerrit.instructure.com/31452
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>