Cdn version append

When taken from the CDN, MathJax will switch the root address from mathjax/latest to mathjax/2.3-latest.  This is to avoid purge and propagation issues with the CDN, and browser cache issues.  We update just MathJax.js (and unpacked/MathJax.js) in mathjax/latest, and when a user gets a new copy of it, it automatically starts getting its file and fonts from the mathjax/2.3-latest URL.  And until the new MathJax.js arrives, the user continues to get v2.2 from mathjax/latest without having a mix of versions as the files propagate to the edge nodes.  Later, when MathJax.js has fully propagated, we can update the rest of the files in mathjax/latest.  For future versions, since only MathJax.js will actually be coming from mathjax/latest, there will be no issue with cached files, since they will be coming from different URLs.