This is a bug fix release that fixes an issue with the latest.js file that is used to obtain the most current 2.x version of MathJax from one of the CDNs that serves MathJax. The problem is that the most current version is only obtained if the highest version on the CDN is version 2.x.y for some x and y, so when MathJax goes to version 3.0, latest.js will find that the current CDN version is 3.0 and (correctly) will not switch to that, but instead will (incorrectly) use the version from which latest.js was loaded rather than the highest 2.x.y available. This means that when version 3.0 is released, sites using latest.js will fall back from version 2.7.5 to the version that they specified for latests.js. MathJax will still run on those pages, but it may be an earlier version than you have been getting in the past.
The new version of latest.js properly handles the situation when the current version is not 2.x, so switching to 2.7.6/latest.js should mean you will get the highest 2.x version in the future, even when v3.0 is released. This version also updates the CDN list to include additional servers, and to remove RawGit, which is no longer in service.
We recommend anyone using latest.js switch to this version.