Use `update-nag` Instead of `updated` Class in Admin Notice

When you use the `updated` admin notice it has a green border which usually means something was successful. It doesn't imply that the user has to do anything. We should instead use the `update-nag` class which has a yellow border which is a bit more intuitive.

http://codex.wordpress.org/Plugin_API/Action_Reference/admin_notices

![dashboard_ _nf_ _wordpress](https://cloud.githubusercontent.com/assets/1065372/4265795/046f8f1a-3c61-11e4-801d-e854612a8606.jpg)