Update recommended MathJax config to match actual secure (#139) config

I didn't find where in the code you set the default templates, so opened Settings→Services and copied from there.

Not sure if this actually affects StackEdit security — are the templates only used for exporting or also for preview?
Anyway, exporting with https URLs is a good thing for wherever the exported file will be used.

If only used for export, consider mathjax's [secure CDN URL](http://www.mathjax.org/resources/faqs/#problem-https) instead of stackedit.io — are you as sure stackedit.io will work 10 years from now as the fact mathjax's CDN will still work?
(admittedly their ugly https URL doesn't look so future-proof; but they claim it's "stable and safe to use")