Derive Ord instances for URI and URIAuth if network < 2.4.0

The current Haskell Platform 2012.4.0.0 uses `network` 2.3.1.0, and `Ord` instances for the `Netwrok.URI` type weren't introduced until `network` 2.4.0.0.

This patch conditionally derives `Ord` if the `network` version is less than 2.4.0.0, so that is possible to build the code against the current Haskell Platform.