Don't print crates.io-index URL when Updating
Don't print crates.io-index URL when Updating

Following the lead from PackageId's Display, only display the registry's
URL if it's not the default registry (aka crates.io).

Before:

    $ cargo install lazy_static
        Updating registry `https://github.com/rust-lang/crates.io-index`

After:

    $ dcargo install lazy_static
        Updating registry

tshepang/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,vojtechkral/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,tsurai/cargo,tsurai/cargo,rust-lang/cargo,vojtechkral/cargo,tsurai/cargo,nipunn1313/cargo,vojtechkral/cargo,vojtechkral/cargo,rustyhorde/cargo,tshepang/cargo,rustyhorde/cargo,rust-lang/cargo,tsurai/cargo,eminence/cargo,tsurai/cargo,eminence/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo