Add PHP 8.1 test (#9)

Author: peter279k

.github/workflows/tests.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        php: ['7.2', '7.3', '7.4', '8.0']
+        php: ['7.2', '7.3', '7.4', '8.0', '8.1']
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -21,4 +21,4 @@ jobs:
 
       - name: Run phpunit
         run: |
-          ./vendor/bin/phpunit
+          ./vendor/bin/phpunit