fix appointment_group context updating for rails 3

fixes a problem where the appointment group sub context tries
to validate that it belongs in record.appointment_group.contexts
before the new_contexts have been added

Change-Id: Ib6d93f68e6af4e9e0885410483275cfa47b7e71f
Reviewed-on: https://gerrit.instructure.com/28434
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>