initialize rack-mini-profiler for rails3

fixes: CNVS-15198

Test Plan:

  - Make sure you have Application Profiling permission.
  - Add ?pp=enable to the end of the url.
  - A profiler output will apear in the top left corner of the screen
    showing that it ran for the request.
  - Add ?pp=disable to the end of the url.
  - The profiler output button should be gone.

Change-Id: I6fa2dee3affa388d07c6eccf05c26c0747ddcd79
Reviewed-on: https://gerrit.instructure.com/40312
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>