Version bump to 2.10.0

Including:

* https://github.com/github/hubot/pull/824
* https://github.com/github/hubot/pull/816
* https://github.com/github/hubot/pull/817
* https://github.com/github/hubot/pull/814
* https://github.com/github/hubot/pull/818
* https://github.com/github/hubot/pull/820
* https://github.com/github/hubot/pull/819