Fix OperatingSystem determine method to operator on a class-level

Author: kevinjalbert

lib/git_statistics/utilities.rb

@@ -30,7 +30,7 @@ class OperatingSystem
  }
  OPERATING_SYSTEMS.default = :unknown
 
- def determine(os_name)
+ def self.determine(os_name)
  OPERATING_SYSTEMS.select { |k,_| k =~ os_name }.first
  end
  end