fixed audio looping (currently broken in the last howler 2.0 beta)

`me.audio.play("myTrack", true);` was not working, I realized that when
looking if ticket #473 was still applicable (and yes it is as well, as
the callback is not fired at all)
the bug is also already referenced on howler side :
https://github.com/goldfire/howler.js/issues/227