Drop unneeded polyfill & Firefox prefix.

1) Firefox prefix hasn't been needed since v23 (we're on 26 now, in 2 days on 27)

2) The polyfill for rAF wasn't used at all in the plugin and was contributing significantly to the file size. Similarly, `cancelAnimationFrame` was feature-detected but then not needed at all. I removed the polyfill & other unneeded parts, shaving off 158 bytes of 428 gzipped.