Add the message thread (if provided) into the deployment payload

Flowdock has message threading and exposes the particular thread via a `thread_id` property on the [message user object](https://github.com/flowdock/hubot-flowdock/blob/3c23b0953b2c70749125c7568be3586830c68836/src/flowdock.coffee#L82). This would be useful to have in the deployment payload for notifying back to the original thread.