Fix CDN address for MathJax to work via https

Modern browsers refuse to load http content when accessing pages via https.
Switch to https CDN for MathJax. This should fix process91/redmine_latex_mathjax/issues/6

Signed-off-by: Kai Blin <kai.blin@age.mpg.de>
Merge pull request #7 from kblin/mathjax-cdn-https-fix

Fix CDN address for MathJax to work via https