loosen rails version requirements for gems

fixes: CNVS-12259

test plan:
- run unit tests, run bundle, verify app starts up without rails version
  conflicts

Change-Id: Iddd6167bbb8a4cc61ca015fc979236659e24da73
Reviewed-on: https://gerrit.instructure.com/33126
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>