rails4: use jammit HEAD from github

There's a rails4 fix that isn't available in a published gem version,
yet.

test plan: compile assets, and verify that still works properly in rails3

Change-Id: I24e7815f8325d52764813eab353ae226dcec0d12
Reviewed-on: https://gerrit.instructure.com/41060
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: August Thornton <august@instructure.com>