allow turning *off* Rails 3 via env var

Change-Id: Ie98f6ecbbe5abc5680bac4ea75e47dbe8e2d8e2c
Reviewed-on: https://gerrit.instructure.com/31790
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>