build: clarify that rkt does not work on OS X

From https://github.com/coreos/rocket/issues/158#issuecomment-65279747, @philips said:
> Right now `rkt` is only designed to work on Linux hosts. Since `rkt` directly executes the binaries and doesn't talk to an API having `rkt` on Darwin doesn't have a lot of utility at this point. I will update the build scripts to fail on darwin with a more useful message.