Support update of fetchers that use `hash` instead of `sha256`

[katexochen]

Currently fetchers with a hash field are not updated by this tool:

{ stdenv, fetchgit }:

stdenv.mkDerivation rec {
  name = "foo-${version}";
  version = "2016-07-13";
  src = fetchgit {
    url = "git://midipix.org/slibtool";
    rev = "4f56fd184ef6020626492a6f954a486d54f8b7ba";
    hash = "";
  };
}

❯ update-nix-fetchgit --version
update-nix-fetchgit-0.2.11

❯ update-nix-fetchgit -v test.nix 
Made no updates

In nixpkgs, sha256 is more and more replaced with hash, so it would be super helpful to support this.