beets: Use audiotools backend for replaygain.

Using commands such as mp3gain and aacgain is only the default for
backwards-compatible reasons. However, on Nix(OS), we would have to
either patch those tools into beets or rely on an impurity, so let's
depend on audiotools and also default to that backend.

Of course, there is also a GStreamer backend, but it comes with a hell
of additional dependencies (which not only cover audio files), which is
why I decided against defaulting to GStreamer and package audiotools
instead (in eecd9324

).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>