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 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.