Add instructions to maximize CPU and RAM of built VM

Default Vagrant images don't make very good build servers :)
Vagrant is still provided as gem since `gem install vagrant` still works (for last gem release), but it is not supported anyway.

My idea was that there is no benefit for this change and 'potentially' there can be an issue with that. But as I said it is not blocker definitely.

And to be honest, I've compiled a lot of win extensions thanks to this dev-box and I wasn't limited with old settings. I think **2 CPUs / 1GB RAM** should be better default value.