Enable HTML-rendered TTML in ref player if Firefox is at least v49
Created by: davemevans
Firefox 49 was released a couple of days ago and now has the required functionality to fix #1164 (closed).
This PR adds a test to the existing UA sniffing to check if the Firefox version number is at least 49, and enables HTML-rendered TTML subtitles in that case, as well as the existing cases (ie !Firefox).