Use niv for version management

[rdnetto]

This PR introduces the use of niv to manage nixpkgs and hie versions, as requested in #56.
It also fixes a bug where hie-nix could not be built on systems where (import <nix/config.nix> ).shell resolved to an executable outside the Nix store (e.g. /bin/bash on Sabayon).

[rdnetto]

The build is failing with:

The job exceeded the maximum time limit for jobs, and has been terminated.

Probably because the relevant Nix packages aren't in Cachix. Looks like it ran for 50 min - when I ran nix build locally, it took 3 hours to complete.

Any idea how to proceed here? It looks it was pushing to Cachix, so re-running enough times should get it passing, but I don't see an option to do so on Travis - I'm guessing the permissions are locked down?

EDIT: Ah, I see from #55 that is a recurring problem and re-running in Travis is inadequate. I'll leave building + merging it to you then.