Update for librosa 0.7

Leaving a marker here that the benchmarks should rerun on librosa 0.7.0 (and probably include version numbers more generally).

Quick summary of changes:

- uses soundfile by default now
- falls back on audioread only if libsndfile can't decode (e.g. for mp3, but maybe other codecs depending on your site build of libsndfile)

As an aside, we always had API support for excerpts and seeking. It wasn't terribly efficient because audioread didn't support that universally, but it should be almost no overhead relative to soundfile now. The only additional overhead would be downmixing or resample-on-load, but those shouldn't be included in the benchmarks anyway.