ci: fix nix version and nix channel version

In order to avoid having bad surprises with automatic updates let's
fix the nix channel version. For now the default travis behavior is to
use `nixpkgs-unstable` (see
https://docs.travis-ci.com/user/languages/nix#overview).

diff --git a/.travis.yml b/.travis.yml
index 4f5d09c4f15a36f43af56e2f0d5cd26f001e35e8..22e80b39f6a3aacc7fdcad65f44e6cb3efadd89f 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,10 +1,12 @@
 ---
-language: nix
-
 sudo: false
+language: nix
+nix: 2.2.1
+env:
+  - NIX_PATH=nixpkgs=channel:nixos-19.09
 
 install:
-  - nix-env -i python2.7-ansible python2.7-ansible-lint ShellCheck-0.7.0
+  - nix-env -iA pkgs.python37Packages.ansible-lint pkgs.python37Packages.ansible pkgs.shellcheck -f '<nixpkgs>'
   - nix-env -if ./dhall-1.26.1.nix
 
   # Check ansible version