[product: zaphod][project: zaphod-v0.3]: Modify README for VirtualBox bug workaround.

SYNOPSIS
[modified]README: VirtualBox 4.3+ must be used on Mac OS X Mavericks hosts.

DETAILS
Older versions of VirtualBox fail to start VMs that use the bridged
AirPort adapter on Mac OS X Mavericks. This problem has been fixed in
version 4.3+ of VirtualBox. The README has been modified to note this
issue in the pre-requsites for installing zaphod on Macs.