HEADS UP: rust-crossbeam-* changed license


I'm updating rust-crossbeam-channel from 0.3.8 to 0.3.9 which has
changed license a bit: `MIT or ASL 2.0` → `(MIT or ASL 2.0) and BSD`.

Other crossbeam crates will have this change as well when upstream
releases new version.

It seems that it should have been there for long time due to some code
taken from "Bounded MPMC queue".