Use cdnjs for mathjax
Use cdnjs for mathjax

Now that the mathjax CDN is deprecated, it's time to switch to cdn.js.
See http://docs.mathjax.org/en/v2.7-latest/start.html#using-a-content-delivery-network-cdn

lsst-sqre/documenteer,lsst-sqre/documenteer,lsst-sqre/sphinxkit