Update to asciicast v2 file format

Thanks for the plugin.

Please update the plugin to use the latest version of asciicast v2 file format that is used in all recording after 18 Feb 2018 (see Published https://blog.asciinema.org/post/two-point-o/).

I have manually updated the plugin files by replacing the plugin files asciinema-player.css and asciinema-player.js with the ones from
https://github.com/asciinema/asciinema-player/releases

Thanks