* v5. Change audio.vtt to audio.vtt.json to fix loading on iOS8 iBooks.

  Somehow, Howler does not load on iOS8 iBooks so mp3 playing is
  disabled there for now; the old <audio> way did work however,
  so maybe it could be fixed by a Howler setting down the road.

  Also this depends on an yet-unreleased version of react-vtt 0.1.7;
  @caasi please publish it to npm at your convenience.
* Update EPUB