replace Travis CI with GitHub Actions

Author: xabbuh

.github/workflows/ci.yaml

@@ -0,0 +1,33 @@
+name: 'CI'
+
+on:
+  - 'push'
+  - 'pull_request'
+
+jobs:
+  tests:
+    name: 'Validate fixtures'
+
+    runs-on: 'ubuntu-latest'
+
+    strategy:
+      matrix:
+        php_version:
+          - '7.1'
+          - '7.2'
+          - '7.3'
+          - '7.4'
+
+    steps:
+      - name: 'Check out'
+        uses: 'actions/checkout@v4'
+
+      - name: 'Set up PHP'
+        uses: 'shivammathur/setup-php@v2'
+        with:
+          php-version: '${{ matrix.php_version }}'
+
+      - name: 'Install dependencies'
+        run: 'composer update'
+
+      - run: 'php validate.php'