Make newer nix-config work

[Chris Warburton]

I've tried bumping the version of nix-config, to maybe work around the
old fetchgit hash issue. Unfortunately, the new architecture doesn't
seem to fit well with haskell-te.

Perhaps we might split nix-config up into a default.nix which just
returns a set of packages, and have config.nix deal with the
complications of overrides, etc.