updated Vagrantfile

Added dbus-x11 requirement (for FF); increased VM memory

Updated ChromeDriver version

To allow the vagrant user to create the /opt/edx/[log|data|db] folders

Compatibility with the Filesystem Hierarchy Standard: http://www.pathname.com/fhs/

Cf README, this adds support to install and setup a dev instance through
Vagrant. The Vagrantfile at the root handles the vagrant commands like
`vagrant up`.

Provisioning is done by the shell script `vagrant-provisionning.sh`,
which is a light wrapper around the `create-dev-setup.sh` which does
most of the job using the standard process, but limiting possible
side-effect by ensuring the environment on which the install is
performed is clean.

Based on an Ubuntu precise 12.04 LTS.