fix respondus middleware loading in rails 3

it can't go in a to_prepare block, cause the middleware stack has already
been initialized at that point

Change-Id: I6820eb89decf394e6b3acef8ea39556b494ab99d
Reviewed-on: https://gerrit.instructure.com/30150
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>