fix course appointment_participants association for rails 3

activerecord 3 tries to call course[:asset_string] which
returns nil because asset_string is not an attribute

Change-Id: Idb4c5da5ebf4fe2eec4946e991aa091d4bc01ce5
Reviewed-on: https://gerrit.instructure.com/28477
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>