Switch to nixpkgs-unstable channel.

see travis-ci/travis-ci#8483

.travis.yml

@@ -1,5 +1,4 @@
 language: nix
 script:
-    - git clone --depth 1 https://github.com/NixOS/nixpkgs ../nixpkgs
     - nix-build tests/intern.nix
     - nix-build tests/extern.nix

tests/extern.nix

@@ -14,7 +14,7 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-import ./../../nixpkgs/nixos/tests/make-test.nix {
+import <nixpkgs/nixos/tests/make-test.nix> {
 
   nodes =
     { server = { config, pkgs, ... }:

tests/intern.nix

@@ -14,7 +14,7 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-import ./../../nixpkgs/nixos/tests/make-test.nix {
+import <nixpkgs/nixos/tests/make-test.nix> {
 
   machine =
     { config, pkgs, ... }: