remove comment about rails 3 not being ready for production

it kinda is now

Change-Id: Ia21a1bcd8ea321c22eb8334c18638b7bae88e9c5
Reviewed-on: https://gerrit.instructure.com/37434
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>