Update to the new NUR format

See https://github.com/nix-community/NUR/issues/45

README.md

@@ -9,6 +9,7 @@
    [default.nix](./default.nix)
    * Remember to mark the broken packages as `broken = true;` in the `meta`
      attribute, or travis (and consequently caching) will fail!
+   * Library functions, modules and overlays go in the respective directories
 3. Add your NUR repo name and your cachix repo name (optional) to
    [.travis.yml](./.travis.yml)
    * If you use cachix you should also add your cache's private key to travis'