Support Symfony 5

Author: GrahamCampbell

Diff for: composer.json

@@ -24,7 +24,7 @@
         "php-http/message": "^1.2",
         "psr/http-client": "^1.0",
         "psr/http-factory": "^1.0",
-        "symfony/options-resolver": "^3.4 || ^4.0"
+        "symfony/options-resolver": "^3.4 || ^4.0 || ^5.0"
     },
     "require-dev": {
         "guzzlehttp/psr7": "^1.0",