Reference envelope.user if exists to support legacy hubot script

Hi,

I tried using [hubot-cron], however it does not work because fields that expected to be contained in `envelope` are under `envelope.user` object.

I think some of legacy hubot scripts are using `envelope.user` object.

So I added a line to support those legacy hubot scripts.

[hubot-cron]: https://github.com/miyagawa/hubot-cron