[HttpClient] Test with guzzle promises v2

Author: nicolas-grekas

composer.json

@@ -35,7 +35,7 @@
         "amphp/http-client": "^4.2.1",
         "amphp/http-tunnel": "^1.0",
         "amphp/socket": "^1.1",
-        "guzzlehttp/promises": "^1.4",
+        "guzzlehttp/promises": "^1.4|^2.0",
         "nyholm/psr7": "^1.0",
         "php-http/httplug": "^1.0|^2.0",
         "php-http/message-factory": "^1.0",