Fix MathJax CDN URL

According to the [MathJax Blog](http://www.mathjax.org/changes-to-the-mathjax-cdn/), MathJax will retire the `rackcdn` CDN address at the end of this month. Updated the layout to use the new CDN URL, and also made it so that the URL is HTTP/HTTPS agnostic, following the [MathJax docs](http://docs.mathjax.org/en/latest/start.html#secure-access-to-the-cdn).