Fix commands module.

1- Add missing module which Command module uses it to get which OS the user is running on. require 'os'
2- Change Mac OS to OS X because os module returns OS X when user is running darwin and not Mac OS.
Note : It's based on #60 pull request which has merged into pickhardt:dev
#60 pull request URL: https://github.com/pickhardt/betty/pull/60
#60 pull request desc: Fix regex. Now it shows OS X and other Oses name correctly.