Declare reformatter dependency in nix-mode.el

[chvp]

Since #176 nix-mode is broken for me. Only the dependencies in the main nix-mode.el file seem to be picked up, so this should fix it again.

[bendlas]

I've managed to work around this bug on my setup with monkey-patching:

        (updatePackage "nix-mode" (epkgs: old: {
          propagatedBuildInputs = old.propagatedBuildInputs ++ [ epkgs.reformatter ];
          propagatedUserEnvPkgs = old.propagatedUserEnvPkgs ++ [ epkgs.reformatter ];
        }))

It would be very nice to get this fixed upstream, @matthewbauer

[etu]

I hope nobody minds if I just merge this... I'm not active in this project really but I have the ability to merge and my nix-mode is currently totally broken... as is everyone else's unless you happen to have reformatter installed for other reasons.