fix external mathjax script, fixes CNVS-14540

cdn was giving 404s; this was causing a selenium spec failure and
preventing preview from working in the advanced editor. switch to
something more reliable.

from my limited testing, we can't easily host this ourselves, because it
dynamically loads other scripts.

test plan:
1. go to the equation editor
2. confirm there are no errors in the js console
3. confirm that the basic editor works
4. confirm that the advanced editor works (live preview too)

Change-Id: Id261740d848cdae8922c9cfd985c9c7e7f84236e
Reviewed-on: https://gerrit.instructure.com/38624
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>