Create ci.yml (#15)

* Create ci.yml
* Allow newer PHPUnit

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+on:
+  - pull_request
+  - push
+
+name: CI
+
+jobs:
+  run:
+    name: Tests
+
+    strategy:
+      matrix:
+        operating-system: [ubuntu-latest]
+        php-versions: ['7.1', '7.2', '7.3', '7.4']
+
+    runs-on: ${{ matrix.operating-system }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+
+      - name: Install dependencies with composer
+        run: composer install
+
+      - name: Run tests
+        run: ./vendor/bin/phpunit

composer.json

@@ -22,7 +22,7 @@
 		}
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^7.5",
+		"phpunit/phpunit": "~7.5|~9.3",
 		"donatj/drop": "^1.0"
 	}
 }