add specialised mathjax CDN URL for readthedocs

RTD serves documentation over HTTPS which means that Chrome refuses to
load JavaScript over HTTP. When compiling under RTD, change the MathJax
CDN URL as noted in [1].

[1] http://docs.mathjax.org/en/latest/start.html#secure-access-to-the-cdn
docs: fix comment on custom MathJax URL

The comment implied the URL was protocol-less but in fact it explicitly
specifies HTTPS.
Merge pull request #45 from rjw57/mathjax-cdn

workaround enabling MathJax under ReadTheDocs