Fix broken URL for MathJAX Javascript dependency.

Loaded using an DNS alias for the CDN, as per the
[instructions](http://docs.mathjax.org/en/latest/configuration.html)

I also switched from `latest` to `2.3-latest`, so that we don't
automatically perform major upgrades.
Merge pull request #3967 from retronym/topic/mathjax

Fix broken URL for MathJAX Javascript dependency.