Add virtualbox package and install vagrant manually

This is necessary because the Ubuntu APT repository contains
an obsolete version of Vagrant that is not compatible with
later versions of VirtualBox. We install the .deb by hand.
Add web browser packages

Baseline of Firefox and Chromium. No way to test IE on Linux without
virtualization, but generally I find that a CSS normalization file
and testing on Gecko and Webkit browsers will turn out okay in IE.