Update rlp requirement from 0.5 to 0.6

Updates the requirements on [rlp](https://github.com/paritytech/parity-common) to permit the latest version.
- [Commits](paritytech/parity-common@rlp-v0.5.0...rlp-v0.6.1)

---
updated-dependencies:
- dependency-name: rlp
  dependency-version: 0.6.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

ethcontract-mock/Cargo.toml

@@ -15,7 +15,7 @@ Tools for mocking ethereum contracts.
 ethcontract = { version = "0.25.9", path = "../ethcontract", default-features = false, features = ["derive"] }
 hex = "0.4"
 mockall = "0.11"
-rlp = "0.5"
+rlp = "0.6"
 predicates = "3.0"
 
 [dev-dependencies]

ethcontract/Cargo.toml

@@ -43,7 +43,7 @@ hex = "0.4"
 jsonrpc-core = "18.0"
 lazy_static = "1.4"
 primitive-types = { version = "0.12", features = ["fp-conversion"] }
-rlp = { version = "0.5", default-features = false, optional = true }
+rlp = { version = "0.6", default-features = false, optional = true }
 secp256k1 = { version = "0.27", features = ["recovery"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"