Change PHP 7 constraint to allow 7.3

Author: Fulmenef

composer.json

@@ -7,7 +7,7 @@
     "AFL-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"
   },
   "autoload": {
     "files": [ "registration.php" ],