Update mio requirement from 0.6.14 to 0.7.0

Updates the requirements on [mio](https://github.com/tokio-rs/mio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/mio/releases)
- [Changelog](https://github.com/tokio-rs/mio/blob/master/CHANGELOG.md)
- [Commits](tokio-rs/mio@v0.6.14...v0.7.0)

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

rustls-mio/Cargo.toml

@@ -23,7 +23,7 @@ webpki = "0.21.0"
 ct-logs = "0.6"
 docopt = "~1.1"
 env_logger = "0.7.1"
-mio = "0.6.14"
+mio = "0.7.0"
 regex = "1.0"
 serde = "1.0"
 serde_derive = "1.0"