Add instructions to maximize CPU and RAM of built VM

Default Vagrant images don't make very good build servers :)
@vassilevsky I wouldn't consider those *fixes* but more as styling decisions. I personally use 1.9-hash style, but always double quotes for strings.

Vagrant installers has been bundling Ruby 1.9.3 since 1.5 release I believe, which defeats any complains about using 1.9-hash style on Ruby 1.8 (Vagrant is no longer provided as a gem or anything).

I've no personal attachment to this change of style, but is not me which decides if this gets merged or not, is @tjschuck (hint hint) :smile: