spec: fix content_migrations_spec rails 3 stability
spec: fix content_migrations_spec rails 3 stability

Change-Id: Idfdde185ead6c615867582ad45aa44b7aa9c87cc
Reviewed-on: https://gerrit.instructure.com/36630
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Shawn Meredith <443646d156d0e0db4e143fd54251ca15b42bcccb@instructure.com>
Reviewed-by: Bryan Madsen <0a3ba1af216659500dd8f41e29f3979f42452870@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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