change find_existing_participant for rails 3 compatibility

otherwise it adds it to the association and tries to save it

Change-Id: I542813b52deb3e13b7317b6258534161769e61b0
Reviewed-on: https://gerrit.instructure.com/29430
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>