add all supported PHP versions to test action

Author: Gummibeer

.github/workflows/run-tests.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [7.4]
+        php: [7.1, 7.2, 7.3, 7.4]
         dependency-version: [prefer-lowest, prefer-stable]
 
     name: P${{ matrix.php }} - ${{ matrix.dependency-version }}