package.json: bump ws dependency to ^0.6.4 with subprotocol fix

See:
  https://groups.google.com/d/msg/sip_js/5GyRZHvyUTI/QFOkKhx8tUQJ
  https://github.com/einaros/ws/pull/415
  https://github.com/einaros/ws/commit/c7f1b4eb44ce45f152f1923b4ba18446f83d0dff