Update php-http/guzzle6-adapter dev dependency to ^2.0

Using ^1.1 prevents true PSR-18 support when installing dev dependencies.

Author: JaZo

composer.json

@@ -30,7 +30,7 @@
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.14",
-        "php-http/guzzle6-adapter": "^1.1",
+        "php-http/guzzle6-adapter": "^2.0",
         "phpunit/phpunit": "^6.0"
     },
     "provide": {