In your JavaScript console, run:

  var s = document.createElement('script');
  s.src = 'https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML';
  document.body.appendChild(s);

The page is pointing to the old MathJax CDN, which was decommissioned on July 31: http://www.mathjax.org/changes-to-the-mathjax-cdn/