Add unicorn duplicate task

When using the [Github recommended procedure](https://github.com/blog/517-unicorn) for unicorn zero downtime deployment also mentioned in this [rails cast](http://railscasts.com/episodes/373-zero-downtime-deployment?view=asciicast), You want the old unicorn master quitting to be handled by the "before_fork" hook in the unicorn.rb config file. In order for us to do that we need a task that won't kill the old master just duplicate it.