ci: stable -> unstable

NUR upstream requires that everything builds on Nixpkgs unstable, so I
guess that's what we're using. Also enable NixOS unstable to see if we
can get that for free as well.

.travis.yml

@@ -12,9 +12,9 @@ env:
 
 jobs:
   include:
-    # - env: NIX_PATH=nixpkgs=channel:nixpkgs-unstable
-    # - env: NIX_PATH=nixpkgs=channel:nixos-unstable
-    - env: NIX_PATH=nixpkgs=channel:nixos-20.03
+    - env: NIX_PATH=nixpkgs=channel:nixpkgs-unstable
+    - env: NIX_PATH=nixpkgs=channel:nixos-unstable
+    # - env: NIX_PATH=nixpkgs=channel:nixos-20.03
 
 script:
  - nix-build ci.nix -kA buildOutputs