Add the "std" feature to the futures-io dependency

Otherwise AsyncRead/AsyncWrite are not available.

Fixes https://github.com/sdroege/async-tungstenite/issues/18

Cargo.toml

@@ -27,7 +27,7 @@ features = ["async-std-runtime", "tokio-runtime", "gio-runtime", "async-tls", "a
 [dependencies]
 log = "0.4"
 futures-util = { version = "0.3", default-features = false, features = ["async-await", "sink", "std"] }
-futures-io = { version = "0.3", default-features = false }
+futures-io = { version = "0.3", default-features = false, features = ["std"] }
 pin-project = "0.4"
 
 [dependencies.tungstenite]