Do not post to the flow if capitrano is in dry-run mode

Without this diff, a successful deployment notification is sent to the flow even if Capistrano runs in dry_run mode (the `-n` flag). 
In the context of this capistrano task, `dry_run` is always defined at Capitrano's level, so this diff should never cause a deploy to abort (see https://github.com/capistrano/capistrano/blob/master/lib/capistrano/configuration.rb#L24)

I'm proposing this fix because using flowdock's Capistrano task without it can cause great confusion in a team, leading people to believe that a new version has been deployed when it was not.