GitHub Actions: added PHP 7.3 & 7.4

Author: janpecha

.github/workflows/tests.yml

@@ -7,7 +7,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             matrix:
-                php: ['5.6', '7.0', '7.1', '7.2']
+                php: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4']
 
             fail-fast: false