Remove unicorn_rails -P {pid} flag

Unicorn_rails does not use the -P flag to specify the location of its pid file
(it uses the accompanying configuration file). Unfortunately, including the -P
flag was causing `ENV['RAILS_RELATIVE_URL_ROOT']` to be overwritten with the
path to the unicorn pid (which is nonsensical).
Merge pull request #20 from zpencerq/master

Remove unicorn_rails -P {pidfile} flag