Revert the Ruby version on windows to 2.0.0-p451

We need this in order to unblock Chef DK release due to https://github.com/ffi/ffi/issues/375.

/cc: @opscode/client-engineers 

I've manually verified this and also running http://manhattan.ci.opscode.us/job/chefdk-test/388/ (which hit an intermittent Mac test issue)