Adjust check for maintenance status

<img width="1027" alt="bildschirmfoto 2018-03-02 um 09 40 41" src="https://user-images.githubusercontent.com/617637/36890963-33ff2036-1e00-11e8-93f0-e5b58c331055.png">

Announcement: https://make.wordpress.org/plugins/2018/02/13/not-updated-in-warning/

Instead of checking the `last_updated` field we should now compare the `tested` field against the latest stable WordPress version.