Adjusting autoplay to properly wait for loadstart event.

Since moving the loadstart into the `player.ready` function [here](https://github.com/videojs/video.js/commit/bad5130c8ff76c96494f0918399a1976dc1a2278), there
exists a race condition whereby the player emits a `play` event
before the `loadstart` event, but since `onLoadStart` was waiting
for that `play` event, the play button never goes away.

alternatively, we could adjust https://github.com/videojs/video.js/blob/master/src/js/player.js#L412-L413 to instead wait for the `playing` event instead of the `play` event, since we know that the `playing` event will always be after the `loadstart`, whereas `play` is not guaranteed to be so.