Update async-tungstenite requirement in /gremlin-client

Updates the requirements on [async-tungstenite](https://github.com/sdroege/async-tungstenite) to permit the latest version.
- [Release notes](https://github.com/sdroege/async-tungstenite/releases)
- [Changelog](https://github.com/sdroege/async-tungstenite/blob/master/CHANGELOG.md)
- [Commits](sdroege/async-tungstenite@0.12.0...0.15.0)

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

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

gremlin-client/Cargo.toml

@@ -42,7 +42,7 @@ chrono = "0.4"
 lazy_static = "1.3.0"
 base64 = "0.12.1"
 native-tls = "0.2.3"
-async-tungstenite = { version = "0.12", optional = true, default-features=false}
+async-tungstenite = { version = "0.15", optional = true, default-features=false}
 async-std =  { version = "1.4.0", optional = true, features = ["unstable","attributes"] }
 async-trait = { version = "0.1.10", optional = true }
 async-tls =  { version = "0.11", optional = true }