Upgrade katex

Summary:
Update the included katex version to v0.1.1. This also involved changing the
files included in the main `.html` files. Also substate khan-exercises
and react-components to the new versions.

Test Plan:
 - Ensure that new math like $\left(\dfrac{x^2}{y}\right)$ works
 - Ensure that MathJax still works by testing $\sqrt[3]{x}$.
 - Add all the widgets and try adding math to all the text areas
   - Ensure that if math is supposed to appear, it appears

Reviewers: alpert, aria

Reviewed By: alpert, aria

Differential Revision: http://phabricator.khanacademy.org/D14304