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).
Update --allow-uploads help text in README
Update README.md with installation instructions
Use Asciidoctor 1.5.0 new syntax backtick

Asciidoctor 1.5.0 is now using backtick for inline code.
Replaces ASCIIDoc by AsciiDoc to respect the case.