Add client-common dependency

Author: ddeboer

composer.json

@@ -13,6 +13,7 @@
     "require": {
         "php": ">=5.4",
         "php-http/httplug": "^1.0@beta",
+        "php-http/client-common": "~0.1",
         "php-http/discovery": "^0.5"
     },
     "require-dev": {