Call MathJax over https

## Call MathJax over https

#### What?
A very simple change: instead of calling MathJax with `http://...`, now is called over `https://...`.

#### Why?
Loading MathJax over `http` triggered security warnings in Chrome and flat-out refused loading in the default behavior in Firefox, unless the user thought about clicking the shield icon next to the address and choosing to allow loading from insecure sources. Let's say that this is less than ideal.

#### Fixed?
Yes, according to my testing at least. A very simple fix that now doesn't trigger any warnings for the end user. Also, extra security afforded by the `https` protocol.

#### Credit
... were credit is due. Thanks to *Julien Bouquillon* aka [revolunet][1] for the awesome package & to *Brian P. Schmidt* aka [bps10][2] for the package's MathJax support. 

[1]: https://github.com/revolunet "revolunet"
[2]: https://github.com/bps10 "bps10"