sudo timeouts during installs

Hi there,

Having run dev-setup on a few machines now (thanks! loving it so far) I've noticed that installs require typing the sudo password at various points, making unattended installs impossible.

A solution we used in [Boxen](https://github.com/boxen/boxen-web/blob/master/app/views/splash/script.sh.erb) for this problem was to ask for the user's sudo password at the beginning (using `sudo -p`) and periodically call `sudo -p` to prevent timeout or prompt the user again. Happy to put up a PR if you're interested.