Add GitHub CI (#79)

* Create main.yml

* Update main.yml

.github/workflows/main.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:    
+      - v*.*
+
+jobs:
+
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php: [7.1, 7.2, 7.3, 7.4]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v1
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v1
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+
+      - name: Validate composer.json
+        run: composer validate --strict
+
+      - name: Install dependencies
+        run: composer update --prefer-dist --no-interaction --no-suggest --no-progress
+
+      - name: Execute tests
+        run: php vendor/bin/phpunit