Update message-io requirement from 0.17.0 to 0.18.0 (#384)

Updates the requirements on [message-io](https://github.com/lemunozm/message-io) to permit the latest version.
- [Changelog](https://github.com/lemunozm/message-io/blob/master/CHANGELOG.md)
- [Commits](lemunozm/message-io@v0.17.0...v0.18.0)

---
updated-dependencies:
- dependency-name: message-io
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

proptest-state-machine/Cargo.toml

@@ -27,4 +27,4 @@ proptest = { version = "1.2.0", path = "../proptest", default-features = true, f
 ] }
 
 [dev-dependencies]
-message-io = "0.17.0"
+message-io = "0.18.0"