spec: fix delayed_message_spec for rails 3 compatibility

Change-Id: Ib21203265ef688e07b11ab24030c48612d23f13e
Reviewed-on: https://gerrit.instructure.com/29427
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>