Selectmenu: Remove a call to the deprecated .focus() method

Replaces a call to the deprecated jQuery `.focus()` method
with `.trigger("focus")`.

Closes gh-2053

ui/widgets/selectmenu.js

@@ -417,7 +417,7 @@ return $.widget( "ui.selectmenu", [ $.ui.formResetMixin, {
 		// Support: IE
 		// Setting the text selection kills the button focus in IE, but
 		// restoring the focus doesn't kill the selection.
-		this.button.focus();
+		this.button.trigger( "focus" );
 	},
 
 	_documentClick: {