Fix mathjax on website broken due to https

The dealii.org webserver now tries to serve https sites. This breaks mathjax because of:

Mixed Content: The page at 'https://www.dealii.org/developer/doxygen/deal.II/step_31.html' was loaded over HTTPS, but requested an insecure resource 'http://cdn.mathjax.org/mathjax/latest/MathJax.js'. This request has been blocked;

This commit fixes this by loading it over https.