Update message factory dependency

Author: sagikazarmark

composer.json

@@ -13,7 +13,7 @@
     "require": {
         "php": ">=5.4",
         "php-http/httplug": "^1.0.0-alpha2@dev",
-        "php-http/message-factory": "^0.3"
+        "php-http/message-factory": "^0.4@dev"
     },
     "require-dev": {
         "phpspec/phpspec": "^2.2",