Workaround for facter os major version thing

I've tested this in Centos 6.5 and it works. Not sure about an older OS that uses the old fact name, but this should retain the old behavior in that case.
This is separate functionality and should be in a different commit. For that, please add the new option to the README and write tests that show what should happen by default, when you specify a valid value, and when you specify an invalid value it should fail. For that last part, you need to validate the parameter.