remove rails 2 support

closes #CNVS-14261

Change-Id: I1b0e8a4438ab659ecd1ac8462bc3559d810989b3
Reviewed-on: https://gerrit.instructure.com/38744
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>