List postgresql as required

Without this, and assuming postgresql has yet to be installed, then `script/bootstrap` fails when building the `pg` gem native extension. The error doesn't make the cause immediately obvious.

I found myself recently bootstrapping a new boxen repo. With a pristine, uncustomized boxen install, and following the *Distribution* section of the `our-boxen` docs leads one here, it seems reasonable to account for not having postgresql installed.