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 script to used 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).
Merge pull request #841 from marcusps/mathjax-cdn-fix

Fix MathJax CDN URL
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Merge branch 'master' of https://github.com/syon/gollum