Fix user name of who updates items on Qiita:Team

Currently,  idobata shows name of creator of document even when someone who is not creator of document updates document. This happens for shared documents (共同編集).

To show correct user name, information in `user` payload should be used according to [Qiita Webhook document](http://qiita.com/docs/webhook). 

This PR fixes to use information in 'user' payload.