base: missing MathJax config variable
base: missing MathJax config variable

* Adds missing config variable CFG_MATHJAX_RENDERS_MATHML to default
  configuration.

Signed-off-by: Lars Holm Nielsen <7eaffca9f7cd98839b5821d0d23e09ff764b82cd@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio