Update pretty_env_logger requirement from 0.3 to 0.4

Updates the requirements on [pretty_env_logger](https://github.com/seanmonstar/pretty-env-logger) to permit the latest version.
- [Release notes](https://github.com/seanmonstar/pretty-env-logger/releases)
- [Commits](seanmonstar/pretty-env-logger@v0.3.0...v0.4.0)

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

examples/websocket/Cargo.toml

@@ -11,6 +11,6 @@ gotham = { path = "../../gotham" }
 futures = "0.3.1"
 tokio-tungstenite = "0.10"
 tokio = "0.2"
-pretty_env_logger = "0.3"
+pretty_env_logger = "0.4"
 sha1 = "*"
 base64 = "*"