Add psql to vagrant script 

- Addresses a pg error by addding postrgesql to the install script based on his [vagrant script for LocalSupport](https://github.com/AgileVentures/LocalSupport/blob/develop/vagrant-ubuntu-install.sh)
- Also adds unzip for easy extraction of homework skeletons

This PR is duplicated on [saasbook/courseware](https://github.com/saasbook/courseware/pull/9) in order to expedite changes.