ci: switch from Travis CI to GitHub Actions

Unfortunately Travis CI seems stuck on Nix 2.0 on macOS, which no
longer works with Nixpkgs unstable.

.github/workflows/test.yml

@@ -0,0 +1,17 @@
+name: "Test"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v10
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - run: nix-shell . -A install
+    - run: nix-shell --pure --max-jobs 4 tests -A run.all

.travis.yml

@@ -1,9 +0,0 @@
-language: nix
-
-os:
-  - linux
-  - osx
-
-script:
-  - nix-shell . -A install
-  - nix-shell --pure --max-jobs 10 tests -A run.all