Use `JsShrink` if `jsxmin` is not available

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