Updated CI to use NixOS 18.03

Author: rdnetto

.travis.yml

@@ -29,7 +29,7 @@ jobs:
       - curl https://nixos.org/nix/install | sh
       - source ~/.nix-profile/etc/profile.d/nix.sh
       # nixpkgs-unstable not supported
-      - nix-channel --add https://nixos.org/channels/nixos-17.09 nixpkgs
+      - nix-channel --add https://nixos.org/channels/nixos-18.03 nixpkgs
       - nix-channel --update
       script:
       - nix-env -i cabal-install