Added PHP 7.3 in composer.json (#14)

Author: ruthgeridema

composer.json

@@ -7,7 +7,7 @@
     "OSL-3.0"
   ],
   "require": {
-    "php": "~5.5.0|~5.6.0|~7.0.0|~7.1.0|~7.2.0",
+    "php": "~5.5.0|~5.6.0|~7.0.0|~7.1.0|~7.2.0|~7.3.0",
     "magento/magento-composer-installer": "*",
     "magento/framework": ">=100.1.0",
     "magento/module-newsletter": ">=100.0",