Update katex to the latest version

Summary:
Substate katex with the v0.1.1 version. Change all of the places KaTeX
are used:

 - Change how the `processMath` function works to use the `katex.render`.
 - Update the Ruler to use the new font name

Test Plan:
 - Open a bunch of exercises
 - Ensure that all of the math looks reasonable

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D14302