Update snow requirement from =0.3.1 to =0.4.0 (#934)
Update snow requirement from =0.3.1 to =0.4.0 (#934)

Updates the requirements on [snow](https://github.com/mcginty/snow) to permit the latest version.
- [Release notes](https://github.com/mcginty/snow/releases)
- [Commits](https://github.com/mcginty/snow/commits/v0.4.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
exonum/exonum,alekseysidorov/exonum,alekseysidorov/exonum,alekseysidorov/exonum,alekseysidorov/exonum,exonum/exonum,exonum/exonum,exonum/exonum