Symfony 4 compatibility

Author: Nyholm

composer.json

@@ -10,7 +10,7 @@
     ],
     "require": {
         "php": "^5.5 || ^7.0",
-        "symfony/options-resolver": "^2.6 || ^3.0",
+        "symfony/options-resolver": "^2.6 || ^3.0 || ^4.0",
         "php-http/httplug": "^1.0",
         "php-http/message-factory": "^1.0.2",
         "php-http/discovery": "^1.0"