Update hex-literal requirement from 0.3 to 0.4

Updates the requirements on [hex-literal](https://github.com/RustCrypto/utils) to permit the latest version.
- [Commits](RustCrypto/utils@hex-literal-v0.3.0...hex-literal-v0.4.1)

---
updated-dependencies:
- dependency-name: hex-literal
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

ethcontract/Cargo.toml

@@ -49,5 +49,5 @@ web3 = { version = "0.18", default-features = false, features = ["signing"] }
 zeroize = "1.1"
 
 [dev-dependencies]
-hex-literal = "0.3"
+hex-literal = "0.4"
 tokio = { version = "1.6", features = ["macros"] }