Add .ruby-version

Since this gem at the time of thie commit does not support
Ruby 2.0 (due to the dependency on Grit), this will aid any
contributors using a Ruby version manager.
Less ask, more tell

Use a factory-like approach in determining which flags to
pass to Utilities#get_modified_time.
Introduce OperatingSystem class

To avoid the use of a case statement in asking lots of questions,
use the factory pattern to have a tell-don't-ask style of
determining the systems operating system.