Allow deprecated usage of Error::cause

std::io::Error hasn't implemented `Error::source`, so we can't use that
yet.

src/error.rs

@@ -17,7 +17,8 @@ impl error::Error for PathError {
     fn description(&self) -> &str {
         self.err.description()
     }
-
+
+    #[allow(deprecated)]
     fn cause(&self) -> Option<&error::Error> {
         self.err.cause()
     }