Remove unused vendor/assets/images/video-js.png.

As far as I can see this file is no longer used. It looks as if video.js moved
from using a sprites image file to font icons in the change from v3 to v4 [1].

[1] https://github.com/videojs/video.js/pull/644/files#diff-1
Add instructions on updating video.js version.
Upgrade video.js to v4.6.1.

I followed the instructions in the "Update from video.js release" section
of the README.
Update version to 4.6.1.
Merge pull request #12 from Futurelearn/upgrade-video-js-to-v4.6.1

Upgrade video.js to v4.6.1