Use Nix for CI builds
Use Nix for CI builds

Installing Stack on Travis failed. This script was very old, and things
tend to break after a while if you don't use Nix. So let's use Nix
everywhere. Builds are going to take a very long time then. Caching the
Nix store will fail I believe, I ran into that with Miniserver. It was
something related to permissions if I recall correctly. I should look
into Cachix.

ruuda/hoff,ruuda/hoff