Ditch Travis CI and switch to GitHub actions

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: CI
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+  workflow_dispatch:
+jobs:
+  Tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install dependencies
+        run: |
+          pip install jsonschema yamllint
+      - name: Run tests
+        run: |
+          make lint