Use std::error::Error::source; cause was deprecated in Rust 1.33.

src/error.rs

@@ -79,7 +79,7 @@ impl error::Error for Error {
         }
     }
 
-    fn cause(&self) -> Option<&error::Error> {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match *self {
             Error::InvalidFormat(_) => None,
             Error::Io(ref err) => Some(err),