chore: set rust-version in Cargo.toml (#664)

Author: atouchet

Cargo.toml

@@ -19,6 +19,7 @@ keywords = ["http", "async", "non-blocking"]
 categories = ["asynchronous", "web-programming", "network-programming"]
 exclude = ["fixtures/**", "ci/**"]
 edition = "2018"
+rust-version = "1.56"
 
 [features]
 # Enables `futures::Stream` implementations for various types.