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
Update --allow-uploads help text in README
Merge pull request #853 from joscarsson/patch-1

Update --allow-uploads help text in README
Update README.md with installation instructions
Update README.md

GitHub Markup is now using asciidoctor
Merge pull request #859 from Mogztter/patch-1

Update installation procedure for AsciiDoc
Merge branch 'master' into rc