extensionSystem: Fix rebasing of extensions

We need to remove the extension from the order after it's disabled

https://bugzilla.gnome.org/show_bug.cgi?id=662704

js/ui/extensionSystem.js

@@ -253,6 +253,8 @@ function disableExtension(uuid) {
         }
     }
 
+    extensionOrder.splice(orderIdx, 1);
+
     meta.state = ExtensionState.DISABLED;
     _signals.emit('extension-state-changed', meta);
 }