Add laravel 5.3 support

Author: vinkla

composer.json

@@ -11,8 +11,8 @@
     ],
     "require": {
         "php": "^5.5.9 || ^7.0",
-        "illuminate/contracts": "5.1.* || 5.2.*",
-        "illuminate/support": "5.1.* || 5.2.*",
+        "illuminate/contracts": "5.1.* || 5.2.* || 5.3.*",
+        "illuminate/support": "5.1.* || 5.2.* || 5.3.*",
         "graham-campbell/manager": "^2.3",
         "m4tthumphrey/php-gitlab-api": "^7.13"
     },