Two style tweaks to the update nag so it blends well with help/option panels and the admin bar.

 * Add a top border to it, but hide it under the admin bar. Helps appearance as the page is rendered (as admin bar is last).
 * Bring the update nag slightly off center again (see [19130]) to ensure it lines up with help/options panels.
see #18197.


git-svn-id: https://core.svn.wordpress.org/trunk@19139 1a063a9b-81f0-0310-95a4-ce76da25c4cd