Update tokio-tungstenite requirement from 0.17 to 0.18 (#606)

Updates the requirements on [tokio-tungstenite](https://github.com/snapview/tokio-tungstenite) to permit the latest version.
- [Release notes](https://github.com/snapview/tokio-tungstenite/releases)
- [Changelog](https://github.com/snapview/tokio-tungstenite/blob/master/CHANGELOG.md)
- [Commits](snapview/tokio-tungstenite@v0.17.0...v0.18.0)

---
updated-dependencies:
- dependency-name: tokio-tungstenite
  dependency-type: direct:production
...

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

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

examples/websocket/Cargo.toml

@@ -9,7 +9,7 @@ edition = "2018"
 [dependencies]
 gotham = { path = "../../gotham" }
 futures-util = "0.3.14"
-tokio-tungstenite = "0.17"
+tokio-tungstenite = "0.18"
 tokio = "1.11.0"
 pretty_env_logger = "0.4"
 sha1 = "0.10"