Update composer.json for Laravel 6 support

Author: ssx

composer.json

@@ -4,7 +4,7 @@
     "type": "library",
     "keywords": [ "laravel", "validation", "ip" ],
     "require": {
-        "illuminate/contracts": "^5.5",
+        "illuminate/contracts": "^5.5|^6.0",
         "symfony/http-foundation": "^3.3"
     },
     "require-dev": {