Fix race condition in Chrome (#547)

When the changeMedia frame loads a new Google Drive video, @yt is still
set from before and moreover @yt.ready is still true, so calling play()
can result in a TypeError if the new embed hasn't loaded yet (this
seemed to happen consistently in Chrome and I was unable to make it
happen in Firefox).

www/js/player.js

@@ -1129,6 +1129,7 @@
     };
 
     GoogleDriveYouTubePlayer.prototype.load = function(data) {
+      this.yt = null;
       this.setMediaProperties(data);
       return this.init(data);
     };