Change prompt order, make Sass and Modernizr false by default

As discussed with @sindresorhus we think it would make sense to avoid developers opting for the kitchen-sink by giving them a lighter set of initial defaults. This PR:

* Moves Bootstrap up to the first prompt option
* Makes Sass and Modernizr false by default

Our plan is to eventually deprecate the 'Sass with Compass' option in favor of grunt-sass as it's significantly faster and we no longer consider Compass to offer as much value as we once did.