Add online feature to yubico

Cargo.toml

@@ -58,7 +58,7 @@ jsonwebtoken = "= 4.0.1"
 u2f = "0.1.2"
 
 # Yubico Library
-yubico= { version = "0.4.0", default-features = false }
+yubico = { version = "=0.4.0", features = ["online"], default-features = false }
 
 # A `dotenv` implementation for Rust
 dotenv = { version = "0.13.0", default-features = false }