load MathJax where browsers don't support MathML

Modernizr can't go in the head (and therefore its HTML5 shim can't work) because the MathML test doesn't like it. Apparently the bleeding-edge version of the MathML test (https://github.com/Modernizr/Modernizr/blob/master/feature-detects/mathml.js) doesn't have this problem, but is not yet released.
ICOT further water filling sentence