update php requirement

Author: michield

composer.json

@@ -28,7 +28,7 @@
         "source": "https://github.com/phpList/base-distribution"
     },
     "require": {
-        "php": "~7.0.0 || ~7.1.0 || ~7.2.0",
+        "php": "~7.0.0 || ~7.1.0 || ~7.2.0 || ~7.3.0 || ~7.4.0",
 
         "phplist/core": "4.0.x-dev",
         "phplist/rest-api": "4.0.x-dev"