Allow symfony v4

Author: norkunas

composer.json

@@ -11,7 +11,7 @@
     ],
     "require": {
         "php": "^5.6 || ^7.0",
-        "symfony/options-resolver": "~2.7|^3.0",
+        "symfony/options-resolver": "~2.7|^3.0|^4.0",
         "php-http/client-implementation": "^1.0",
         "php-http/client-common": "^1",
         "php-http/message": "^1.0"