MathJax v2.7.9

This release updates the Speech-Rule Engine (SRE) that underlies MathJax's accessibility features to version 3.1 and updates the a11y extensions to version 1.6.0 to take advantage of the new SRE version. The upgrade includes new languages (German and French), and access to the Clearspeak rules via the accessibility menu, as well as bug fixes in SRE, and faster conversion of expressions to speech.