use native focus event - fixes shift+tab (broken in jquery 3.4.0) (#2469)

Author: caseyjhol

js/bootstrap-select.js

@@ -2625,7 +2625,7 @@
           changedArguments = null;
         })
         .on('focus' + EVENT_KEY, function () {
-          if (!that.options.mobile) that.$button.trigger('focus');
+          if (!that.options.mobile) that.$button[0].focus();
         });
     },