spec: fix content_migrations_spec rails 3 stability

Change-Id: Idfdde185ead6c615867582ad45aa44b7aa9c87cc
Reviewed-on: https://gerrit.instructure.com/36630
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>