Use Phusion's Docker-friendly Vagrant base box.

This PR swaps the default `precise64` Vagrant box for Phusion's [Docker-friendly Vagrant box](http://blog.phusion.nl/2013/11/08/docker-friendly-vagrant-boxes/).

Consequently, I've changed the following things:
- Removed the `vagrant plugin ...`, `vagrant vbguest`, and `vagrant reload` commands from the readme, as those are no longer needed. Phusion's box sees to the installation of the proper VirtualBox Guest Additions automatically, and does not need a reboot as no kernel updates are needed.
- Updated the Vagrantfile to use the new box, and removed the Linux kernel packages from the `apt-get` command, as the new box already ships with a Docker-ready kernel.
- I've named the new box `phusion_ubuntu_12_04`, so it is clear we are using the Phusion box as base, which itself is based on Ubuntu 12.04.

Tested on OS/X 10.9 with VirtualBox 4.2.18, which seems to work fine.