Summary: If `jsxmin` is not available, use a pure PHP implementation instead (JsShrink).
Test Plan:
- Ran `arc lint --lintall` on all JS and fixed every relevant warning.
- Forced minification on and browsed around the site using JS behaviors. Didn't hit anything problematic.
Reviewers: vrana, btrahan
Reviewed By: vrana
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D5670
