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.
Merge pull request #24 from mtyaka/mathjax-cdn

Use MathJax 2.4 hosted on cdnjs.