Updated local-only version of kas.js

Summary: KAS has had some updates over the past few weeks, in particular to allow for parsing
of a greater subset of TeX. Since this 'local-only' version is also the version of KAS used in
the mobile app, it should also be upgraded.

Auditors: eater

Test Plan:
- Run khan-exercises.
- Try "Multiplying polynomials" to verify that KAS is still working.
    - In particular, try using \ast as a multiplication symbol, and verify that it both renders and grades.