Select video-js element based on class, not tag name

Fix for #1369.

Tested on WinXP virtualbox in Firefox. Could do with some more testing @bcipolli and @ruimalheiro.

Summary of changes:
* Select video-js element based on the class not on 'video' tag, as it changes to 'object' tag when using flash.

One line fix. Every. Freakin'. Time.