Switch the https path for mathjax

Thanks to robo9k for pointing out the situation in #97. Essentially we
sometimes see the Breathe docs from an https URL and we can't load the
default http mathjax URL from an https view so we switch to an https
mathjax URL which we can load from both http and https.
Acknowledge robo9k's contributions
Acknowledge sieben's contribution
Merge branch 'https'