Specify the minimum required Rust version in the manifest

Also mention it in the deployment guide.

Cargo.toml

@@ -8,6 +8,7 @@ repository = "https://gitlab.com/famedly/conduit"
 readme = "README.md"
 version = "0.1.0"
 edition = "2018"
+rust = "1.50"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html