Unicorn: shut down old master upon first fork

After the first child forks, shut down the old master.
Previously, the old master and it's workers would jut sit there
forever.

See: https://github.com/blog/517-unicorn and
     https://github.com/sosedoff/capistrano-unicorn/blob/master/examples/rails3.rb#L30

Tested in production, works great.