Add Symfony 5 version constraint (php-http#46)

Author: yeclense

composer.json

@@ -13,7 +13,7 @@
         "nyholm/psr7": "^1.0",
         "php-http/httplug": "^2.0",
         "psr/http-client": "^1.0",
-        "symfony/options-resolver": "^2.6 || ^3.0 || ^4.0"
+        "symfony/options-resolver": "^2.6 || ^3.0 || ^4.0 || ^5.0"
     },
     "require-dev": {
         "concurrent-php/async-api": "dev-master",