Add: MathJax 2.7.8

[TomasRiker]

MathJax versions 2.x and 3.x are all present on cdnjs, 2.7.8 is the only one that's missing. Please add it as well.

[MattIPv4]

@tc80 I wonder if the bot should be able to backfill missing versions like this? Maybe on each run we check for the most recent 10 versions and add any that are missing?

[tc80]

sgtm, will make a PR