make Rails 3 the default

Rails2 support still available by env var or config/RAILS2

Change-Id: I127105dfd3d3bef19e51a2c7fc53b64748115625
Reviewed-on: https://gerrit.instructure.com/37471
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>