Merge pull request #523 from hal869/master

enable guzzle6-adapter ^2.0 usage

Author: dbu

composer.json

@@ -28,7 +28,7 @@
         "symfony/http-kernel": "^3.4.4 || ^4.0"
     },
     "require-dev": {
-        "php-http/guzzle6-adapter": "^1.0",
+        "php-http/guzzle6-adapter": "^1.0 || ^2.0",
         "php-http/message": "^1.0",
         "mockery/mockery": "^1.0",
         "monolog/monolog": "*",