spec: fix discussion_topic_spec for rails 3

Change-Id: I8f8839834f880d0c364c881e2687f323ff71d8d9
Reviewed-on: https://gerrit.instructure.com/29431
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>