replace travis with github action

This way we get the latest nix.

.github/workflows/test.yml

@@ -0,0 +1,13 @@
+name: "Test"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v10
+    - name: Show nixpkgs version
+      run: nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
+    - run: ./tests/run.py