switch JSON back-ends

closes CNVS-3410

test plan:
* parsing and emitting json should still work as before -- the API is a
  great way to verify this. Also just using canvas pages that make AJAX
  calls back to the server.

Change-Id: I30ac2aa6b88c51927dbf71067766db0de02b0c45
Reviewed-on: https://gerrit.instructure.com/17206
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>
Tested-by: Brian Palmer <brianp@instructure.com>
treesame commit of origin/stable/2013-01-26

Treesame-Commit-Id: 90378ae9b51b8acf0be690bca61f5f1454f3e0fe