The current version of latest.js looks for the latest version of MathJax on the CDN, and if it is version 2.x for some x, it loads that. If not, it loads whatever version the latest.js was taken from.
When Version 3.0 is released, the current version will be 3.0.0, and so latest.js will load the version where latest.js was loaded rather than the latest 2.x version.
This PR backports the v3.0 latest.js, which resolves this problem (as well as adds a number of other CDN choices). In this version, when the current CDN version isn't version 2.x, it will look up the available versions and take the highest 2.x version.