At this point, you may as well grab it from `uname -r`.

If you're going to base it off `MacOS.version`, better to call it once and use a case statement:

```ruby
case MacOS.version
when 10.4 then 8
when 10.5 then 9
when 10.6 then 10
when 10.7 then 11
when 10.8 then 12
end
```