beets: Use bs1770gain by default for replaygain

After trying with a dozen files, it seems the bs1770gain backend is much
more reliable than the audiotools backend and especially does a better
job (well, compared to audiotools which either does doing nothing at all
or throws an exception) when used on alboms that contain different
sample rates/sizes.

Additionally, we already had a few issues regarding the audiotools
backend, even to the extent that @sampsyco almost wanted to drop it
upstream (see sampsyco/beets#1342).

Also related issues are #10376 and sampsyo/beets#1592.

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