bump rails 3 for security fix

Change-Id: Ic5cb76169c56680546cafe3b3e2d3ca22165104c
Reviewed-on: https://gerrit.instructure.com/38257
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>