Upgrade video.js to v4.6.1

I wanted one of the bug fixes released in v4.6.0 and so I thought I'd have a go at upgrading video.js myself.

I've added some instructions to the README on how to do this in general - if only to document what I've done in case I've missed something - but hopefully also to make it easier for others to do in the future.

I've also removed a file which I think is no longer used by video.js and updated the version of the gem to v4.6.1 to match the video.js version.