rails3: fix RequestContextGenerator middleware order

Make sure it comes before ActionDispatch's logging of the request start

fixes CNVS-13890

test plan: turn on 'log_context: true' in config/logging.yml, and make
sure that the `Started GET "/path"` lines have the correct request id of the
subsequent request, not the previous request.

Change-Id: Ie6a77993955296b9032e07f8536506b6816517b7
Reviewed-on: https://gerrit.instructure.com/37183
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>