Use require to clarify ext.uls.preferences

ext.uls.preferences only depended on ext.uls.common because of its
use of the uls global. This is unnecessary, and made clearer by using
require and module.exports to access it.

Change-Id: Ied2a1b560d19a18529fd766ced778c912199fa2f

tests/qunit/ext.uls.tests.js

@@ -40,7 +40,7 @@
 		// Here it's used as a meaningless word, to test
 		// the preferences without changing anything useful.
 		prefName = 'gofanim';
-		prefs = mw.uls.preferences();
+		prefs = require( 'ext.uls.preferences' )();
 		prefsToSave = {};
 
 		prefsToSave[ prefName ] = {