bump(github.com/coreos/raft): cff0a00

This pull request updates etcd to use the latest raft which includes the following changes:

* Add EventDispatcher.RemoveEventListener() - https://github.com/goraft/raft/pull/177

* Dispatch Commit Event - https://github.com/goraft/raft/pull/178

Tests are running :green_heart:.

/cc: @philips @xiangli-cmu