update notify to remove exception from cargo audit

Currently arti depends on the unmaintained net2, which raises warning with cargo-audit. We can't fix it directly as it's a transitive dependency. Our direct dependency is notify 4.0.17. It's fixed in 5.0.0, however there are only pre-releases (15 of which), with no due date.

Whenever this come out, we should update notify and remove RUSTSEC-2020-0016 from the ignored list