spec: fix selenium/content_migrations_spec for rails 3

Change-Id: I4f73aada8038dc5f882d2a49f631461e23f7c771
Reviewed-on: https://gerrit.instructure.com/30416
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>