[O] Use CHTML mathjax backend by default

[Carlos Pita]

Hi all,

I was having some problems with the mathjax HTML-CSS backend and I
found out that the CHTML backend is currently the preferred one. See
details at https://github.com/mathjax/MathJax/issues/2132 and
http://docs.mathjax.org/en/latest/output.html. Changing the backend to
CHTML fixed my issue.

Wouldn't be time to update the default mathjax version to 2.7.1 (the
current one) and setting the backend to CHTML?

Best regards
--
Carlos