update composer php requirement

composer.json

@@ -2,7 +2,7 @@
     "name": "firegento/magento2-content-provisioning",
     "description": "N/A",
     "require": {
-        "php": "~7.1.3|~7.2.0",
+        "php": "~7.1.3|~7.2.0|~7.3.0",
         "magento/framework": "100.1.*|101.0.*|102.0.*",
         "magento/module-cms": "101.0.*|102.0.*|103.0.*",
         "magento/module-widget": "100.1.*|101.0.*|101.1.*"