Use guzzlehttp instead of whole adapter (#39)

Author: lukasz-falda

composer.json

@@ -10,7 +10,7 @@
     "webmozart/assert": "^1.3",
     "symfony/validator": "^4.1 || ^5.0",
     "symfony/http-kernel": "^4.1 || ^5.0",
-    "php-http/guzzle6-adapter": "^1.1",
+    "guzzlehttp/guzzle": "^6.0 || ^7.0",
     "symfony/serializer": "^4.4 || ^5.0",
     "symfony/property-access": "^4.1 || ^5.0"
   },