added php-http/message as dependency

Author: avigoldman

composer.json

@@ -19,7 +19,8 @@
     "psr/http-message": "^1.0",
     "php-http/httplug": "^1.0",
     "php-http/client-implementation": "^1.0",
-    "php-http/discovery": "^1.0"
+    "php-http/discovery": "^1.0",
+    "php-http/message": "^1.0"
   },
   "require-dev": {
     "php-http/guzzle6-adapter": "^1.0",