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

webroot/rsrc/js/application/differential/behavior-keyboard-nav.js

@@ -130,9 +130,12 @@ JX.behavior('differential-keyboard-navigation', function(config) {
           selection_end = blocks[focus][1];
 
           manager.scrollTo(selection_begin);
-          (refreshFocus = function() {
+
+          refreshFocus = function() {
             manager.focusOn(selection_begin, selection_end);
-          })();
+          };
+
+          refreshFocus();
 
           return;
         } else {