Implement Error::source when supported

[seanmonstar]

The source method is the better replacement for cause. Hyper's Error should implement the method.

However, it's only available on newer Rust versions. To prevent requiring a new version, a build script can check rustc --version and set a cfg flag such that it's only implemented when available.

[sfackler]

The rustc_version crate can handle the heavy lifting in the build script.