Re-add guzzlehttp/psr7

Author: ddeboer

composer.json

@@ -35,6 +35,7 @@
         "monolog/monolog": "~1.0",
         "php-http/guzzle6-adapter": "0.3.x-dev",
         "php-http/mock-client": "0.1.x-dev",
+        "guzzlehttp/psr7": "^1.0",
         "symfony/process": "^2.3||^3.0",
         "symfony/http-kernel": "^2.3||^3.0"
     },