fix redis_spec for rails 3

Change-Id: Idd2a47bd17c191b1827b9a10418d9add2de0eace
Reviewed-on: https://gerrit.instructure.com/30991
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>