Host and Avatar in Slackbot Message

PT Story: https://www.pivotaltracker.com/story/show/78215264

* Sends host_name and host_avatar to slackbot
@sampritipanda -I'm getting old and didn't have my reading specs on :grinning: .

I think we could refactor that method though because its only the s parameter that is added and that is optional. Could we not have something like  ..
```
if options[:size]
  "https://www.gravatar.com/avatar/#{hash}?s=#{options[:size]}&d=retro"
else
  "https://www.gravatar.com/avatar/#{hash}?d=retro"
end
```