Fixes deprecated MathJax URL

Use updated MathJax URL to work in both http and https.

For more information, see:
getpelican/pelican-plugins@389bfdc3ece84c4efe5bd8e3b228f010256ac3e8
and
[Changes to the MathJax CDN](http://www.mathjax.org/changes-to-the-mathjax-cdn/)