rails3: fix message email html

fixes CNVS-12213

test plan:
  * launch Rails 3
  * trigger a notification email
  * view it in /messages
  * verify that it looks the same as Rails 2 emails

Change-Id: I86526453c92e489f5f18002c3fe4adb55ddacf56
Reviewed-on: https://gerrit.instructure.com/32888
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>