Update tokio-util requirement from 0.3.1 to 0.4.0

Updates the requirements on [tokio-util](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Cargo.toml

@@ -12,7 +12,7 @@ edition = "2018"
 
 [dependencies]
 bytes = "0.5.4"
-tokio-util = { version = "0.3.1", features = ["codec"] }
+tokio-util = { version = "0.4.0", features = ["codec"] }
 bit-set = { git = "https://github.com/contain-rs/bit-set", rev ="c27082f" }
 bit-vec = "0.6.2"
 speedy = "0.6.0"