FIX: Non-secure warning from YouTube thumbnails

YouTube thumbnails where being loaded over `http` even on a `https` loaded page.
Causing the browser bar to change from green to yellow ![non-secure content loaded icon](http://i.imgur.com/c4GpwCQ.png)
Merge pull request #2884 from DeanMarkTaylor/patch-2

FIX: Non-secure warning from YouTube thumbnails