remove 19.03 and 19.09 channels

Those are not receiving any more updates. Fix Nix invocations.

.travis.yml

@@ -1,6 +1,6 @@
 language: nix
 
-nix: 2.2.1
+nix: 2.3.6
 
 matrix:
   include:
@@ -8,10 +8,6 @@ matrix:
     env: NIXPKGS_CHANNEL=nixos-unstable IMAGE_TAG=latest
   - name: nixos-20.03
     env: NIXPKGS_CHANNEL=nixos-20.03
-  - name: nixos-19.09
-    env: NIXPKGS_CHANNEL=nixos-19.09
-  - name: nixos-19.03
-    env: NIXPKGS_CHANNEL=nixos-19.03
 
 script:
-  - nix-shell --run ./.travis.sh
+  - NIX_PATH=channel:$NIXPKGS_CHANNEL nix-shell --run ./.travis.sh