fix appointment_group _appointments named scope for rails 3

Change-Id: Icb4d16408fe59980adade489c8e585ec1a87d42c
Reviewed-on: https://gerrit.instructure.com/28459
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>