Update ark-relations requirement from ^0.4.0 to ^0.5.1

Updates the requirements on [ark-relations](https://github.com/arkworks-rs/snark) to permit the latest version.
- [Release notes](https://github.com/arkworks-rs/snark/releases)
- [Changelog](https://github.com/arkworks-rs/snark/blob/master/CHANGELOG.md)
- [Commits](https://github.com/arkworks-rs/snark/commits)

---
updated-dependencies:
- dependency-name: ark-relations
  dependency-type: direct:production
...

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

Cargo.toml

@@ -20,7 +20,7 @@ ark-ec = { version = "^0.4.0", default-features = false }
 ark-serialize = { version = "^0.4.0", default-features = false, features = [ "derive" ] }
 ark-poly = { version = "^0.4.0", default-features = false }
 ark-std = { version = "^0.4.0", default-features = false }
-ark-relations = { version = "^0.4.0", default-features = false }
+ark-relations = { version = "^0.5.1", default-features = false }
 ark-crypto-primitives = { version = "^0.4.0", features = ["snark"], default-features = false }
 ark-r1cs-std = { version = "^0.4.0", default-features = false, optional = true }