Fix ajax error callback is called on abort
Merge pull request #22 from aboudreault/ajax-request-issues

Fix ajax error callback is called on abort
Update MathJax to 2.4.

This fixes some weird infinite scrolling issues on the course discussion
page in IE9 & IE10.

It is probably related to:
https://github.com/mathjax/MathJax/issues/585
Merge branch 'mathjax-upgrade'
Use MathJax 2.4 hosted on cdnjs.

MathJax is a huge dependency. In `edx-platform`,
it is loaded from an S3 bucket hosted by edX (see
https://github.com/edx/edx-platform/blob/bf8183f61e95f655cf4f13d2ee02c0c2c8871a4d/common/templates/mathjax_include.html#L36),
but the version used in `edx-platform` is currently 2.2,
which has some IE bugs that break the discussion layout,
so we can't use that.

The bugs have been fixed in MathJax 2.4.
Remove vendored MathJax.

We are linking to MathJax on cdnjs now,
so no need to vendor in this huge library.