Upgrade to UglifyJS2

Summary: This time, for the exercise packer! This goes along with a Gemfile change in webapp/ to bump the version.

Test Plan: kake/build_prod_main.py -j1 --force khan_exercises

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D5335