Change hubot-yammar adapter

2.4.5からの新しいenvelopeって仕組みに対応してなったので修正版あげた

- https://github.com/github/hubot/pull/388
- https://github.com/hipchat/hubot-hipchat/issues/66
Merge pull request #6 from scubism/feature/change-hubot-yammer

Change hubot-yammar adapter