Pull duration for API from default_audio_version.

Many stories are choosing an audio version that is shorter than the
longest one (by like a minute, in the case of the moth) which means that
the scrubber doesn't work correctly.

This just pulls that duration from the version we are choosing.
correctly calculate duration for promo-only stories with multiple promos
merge branch master
Merge pull request #36 from PRX/fix/duration_from_audio_version

Pull duration for API from default_audio_version.