Merge branch 'develop' into master for 2.7.2-beta.1 release

Merge branch 'patch-a11y'

Merge pull request mathjax#1740 from dpvc/2.7.1-rc.1

Update for 2.7.1

Merge pull request mathjax#1638 from mathjax/develop

v2.7.0 release

Update Contrib to always use https, and repack MathJax.js

Merge pull request mathjax#1382 from mathjax/develop

Merge develop into master for 2.6.1 release

Merge pull request mathjax#1380 from mathjax/issue1346

package.json: bump version to 2.6.1-rc.1

Merge pull request mathjax#1345 from mathjax/develop

Release v2.6.0

Change cache-breaking value in MathJax.js

Repack and recombine for 2.6-beta.1