Allow Symfony 4 (#88)

Author: OndraM

composer.json

@@ -15,7 +15,7 @@
         "php-http/httplug": "^1.1",
         "php-http/message-factory": "^1.0",
         "php-http/message": "^1.6",
-        "symfony/options-resolver": "^2.6 || ^3.0"
+        "symfony/options-resolver": "^2.6 || ^3.0 || ^4.0"
     },
     "require-dev": {
         "phpspec/phpspec": "^2.4",