Don't depend on num's default features; removing rand requirement for build
Don't depend on num's default features; removing rand requirement for build

requirement for rand comes from `num`'s default features which are all unused in `chrono`
not including the default features means that that `build` no longer requires `rand` (and hence does not pass it on to others importing `chrono`)
I thought it should also remove it for `test` / `bench` also but the `serde_json` dev-dependency seems to make it included..
lifthrasiir/rust-chrono,tshakah/rust-chrono