spec: fix rails3 specs in message_spec.rb

Change-Id: Ia844a92b6131c6efecc223cf6b6a4c483369fb7a
Reviewed-on: https://gerrit.instructure.com/29539
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>
QA-Review: Jon Willesen <jonw@instructure.com>