4/16 - Changed selector to just "li.takeover-container".

Using the more specific, multi-class selector (.notification.single_notification.alt.takeover-container.mb_tracked) seems to have a performance hit in Chrome + Stylish, where the ad displays briefly during page load, then once the page finishes loading the display css kicks in to hide it.

However, shortening the selector to the more broad ".takeover-container" solves the delay. I'm just not sure if .takeover-container will be used for any non-ad content on tumblr in the future (right now it only appears twice--the ad--when i view dash source).