[MessageTray] Fix summary to be initially-hidden

Some recent change broke it so that the first time a notification pops
up, the summary will be visible alongside it.

https://bugzilla.gnome.org/show_bug.cgi?id=609765

js/ui/messageTray.js

@@ -361,9 +361,9 @@ MessageTray.prototype = {
                                               x_fill: false,
                                               expand: true });
 
-
         this._summary.connect('enter-event',
                               Lang.bind(this, this._showMessageTray));
+        this._summaryBin.opacity = 0;
 
         this.actor.connect('enter-event',
                            Lang.bind(this, function() {