Update GitHub worflow with PHP7.4

Author: rubenvanassche

.github/workflows/run-tests.yml

@@ -9,7 +9,7 @@ jobs:
         strategy:
             fail-fast: true
             matrix:
-                php: [ 7.2, 7.3]
+                php: [7.2, 7.3, 7.4]
                 dependency-version: [prefer-lowest, prefer-stable]