Fix nightly build

[jasonwhite]

std::io::Error hasn't implemented Error::source, so we can't use that yet. Adding #[allow(deprecated)] is a temporary fix until std::io::Error is updated.

The warning/error that is fixed:

warning: use of deprecated item 'std::error::Error::cause': replaced by Error::source, which can support downcasting
  --> src/error.rs:22:18
   |
22 |         self.err.cause()
   |                  ^^^^^
   |
   = note: #[warn(deprecated)] on by default