Updated Gemfile to contain "open4" gem that is used to retrieve the PID of a process.

open4 is good to use, but it creates risidual zombie processes that won't clear until the Rails Rack process is destroyed.
Added "plug-and-play" functionality in this change. Array of three possible devices. Cycle through each file, test for existence, if it does exist, it'll be my Arduino.
Changed phrasing.
Added a 3-sec sleep to give 'Motion' time to gracefully quit.

Originally used Open4 to kill motion, but because of the zombie processes, I will change this.
Removed the use of Open4 and using the old system call, but I use 'killall motion'. Works well.