Use user.message over thread_id for flowdock adapter

I believe thread_id is from an older implementation and may be deprecated. The message property is more appropriate.

This was my mistake. I should have looked more into the actual values being passed around, but I wrote this implementation based on reading the flowdock adapter source, not running an actual instance. I've now actually run this code and `msg.message.user.message` is the more appropriate value to be using here.