Update predicates requirement from 2.0 to 3.0

Updates the requirements on [predicates](https://github.com/assert-rs/predicates-rs) to permit the latest version.
- [Changelog](https://github.com/assert-rs/predicates-rs/blob/master/CHANGELOG.md)
- [Commits](assert-rs/predicates-rs@v2.0.0...v3.0.3)

---
updated-dependencies:
- dependency-name: predicates
  dependency-type: direct:production
...

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

ethcontract-mock/Cargo.toml

@@ -16,7 +16,7 @@ ethcontract = { version = "0.23.0", path = "../ethcontract", default-features =
 hex = "0.4"
 mockall = "0.11"
 rlp = "0.5"
-predicates = "2.0"
+predicates = "3.0"
 
 [dev-dependencies]
 tokio = { version = "1.6", features = ["macros", "rt"] }