Upgrade dependencies

This bumps all dependencies to the latest versions. If preferred, I
could bump only `rustls` and `webpki-roots`.

Author: Demi-Marie

Cargo.toml

@@ -16,10 +16,10 @@ travis-ci = { repository = "async-std/async-tls" }
 appveyor = { repository = "async-std/async-tls" }
 
 [dependencies]
-futures = "0.3"
-rustls = "0.16"
-webpki = "0.21"
-webpki-roots = "0.17"
+futures = "0.3.4"
+rustls = "0.17.0"
+webpki = "0.21.2"
+webpki-roots = "0.19.0"
 
 [features]
 early-data = []