cabal2nix: update to version 2.0 from Hackage

I've realized that publishing updates to Hackage is far easier than
publishing updates in Nixpkgs, and since all Hackage updates show up in
Nix automatically I've decided to go back to publishing cabal2nix on
Hackage again.

Unfortunately, this means that I'll have to change the version numbering
scheme to comply with the expectations of the Haskell PVP (which is used
by Stackage), so the new version 2.0 looks like a downgrade to Nix,
which used to have version 20160406. :-(

If in doubt, run "nix-env -u --always" to force the update. I am sorry
about the inconvenience.