added platform config php 7.3 to composer.json

composer.json

@@ -78,7 +78,10 @@
     "minimum-stability": "dev",
     "prefer-stable": true,
     "config": {
-        "sort-packages": true
+        "sort-packages": true,
+        "platform": {
+          "php": "7.3"
+        }
     },
     "autoload": {
         "classmap": [