Update urlbar state management to fix Enter key in Nightly

- Upstream bug 1306639 changed how the urlbar state is set. Update how
	we set urlbar state to work with the new approach.

Fixes #322.

lib/ui/highlight-manager.js

@@ -357,7 +357,11 @@ HighlightManager.prototype = {
 
     // Now that we've figured out which item will take the highlight,
     // update the DOM, and fire a signal that sets the newUrl in the urlbar.
-    this.popup.el.selectedIndex = newIndex;
+    if ('setInitiallySelectedIndex' in this.popup.el.input.controller) {
+      this.popup.el.input.controller.setInitiallySelectedIndex(newIndex);
+    } else {
+      this.popup.el.selectedIndex = newIndex;
+    }
     this.recommendation.isHighlighted = shouldHighlightRecommendation;
     this.events.publish('selection-change', { newUrl: newUrl });
   }