Simplify steps

Addressing a pain point and two minor issues:

1. Far down in the instructions it tells me to install Spiff, which does not come with a Windows release. A user cannot be expected to build it themselves, at least not as part of something called 'Lite'. I moved it to the top, with the suggestion to run the spiff and bosh cli steps within the Vagrant VM. Works great!

1. The user does not need to install Rubygems, it's included since Ruby 1.9, and bosh_cli requires >=1.9.3. (I left out the ">=", not sure what Ruby versions are currently recommended/supported by cf and bosh CLIs.

1. The provision_cf script was mentioned in the wrong place: it does not install Spiff and it does download the Warden stemcell (even if it was uploaded already), so moved it to the correct place.