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
     https://github.com/sosedoff/capistrano-unicorn/blob/master/examples/rails3.rb#L30

Tested in production.
Merge pull request #431 from iFixit/unicorn-kill-old-master

Unicorn: shut down old master upon first fork