Merge pull request
#1710 from minrk/mathjaxcdn
update MathJax CDN url for https
As discussed on mathjax-users, the MathJax CDN is moving. This doesn't affect mathjax from http, which just uses cdn.mathjax.org, but for SSL-certiificates the actual CDN url must be used (formerly cloudfront, now rackcdn).
Also, forgo gethostbyname_ex hack, that was me solving a theoretical problem that could not officially happen.
0.12-series is unaffected, as the https-cdn hack was not backported to 0.12.1.
