fix package name of httplug

Author: dbu

composer.json

@@ -12,7 +12,7 @@
     ],
     "require": {
         "php": ">=5.4",
-        "php-http/client": "dev-utility_separation",
+        "php-http/httplug": "dev-utility_separation",
         "php-http/message-factory": "dev-master@dev"
     },
     "require-dev": {