keyboard: Add a missed translation

The word "tray" should be translated to other languages.

https://bugzilla.gnome.org/show_bug.cgi?id=660600

js/ui/keyboard.js

@@ -305,7 +305,8 @@ Keyboard.prototype = {
     },
 
     _getTrayIcon: function () {
-        let trayButton = new St.Button ({ label: "tray", style_class: 'keyboard-key' });
+        let trayButton = new St.Button ({ label: _("tray"),
+                                          style_class: 'keyboard-key' });
         trayButton.key_width = 1;
         trayButton.connect('button-press-event', Lang.bind(this, function () {
             Main.messageTray.toggle();