Allow unicorn:reexec to continue if pid file doesn't exist

* This prevents unicorn:reexec from failing if the pid file doesn't
exist. This fixes an issue we have on initial deployment where are
pid file hasn't been created yet.
* This also allows the pid file to be configurable.
* Using newer $(command) style of command substitution instead of
backticks: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html