Cargo.toml: upgrade dependencies

[kkovaacs]

In particular, tokio isn't pinned to 1.15 now. Pinning tokio was making it hard for us to use goose in projects which use other dependencies that require a newer version of tokio.

[jeremyandrews]

Please see https://github.com/tag1consulting/goose/pull/420 -- there's a known issue preventing this update, as tests fail. The work in https://github.com/tag1consulting/goose/pull/509 will solve this once it lands.