Add a Vagrantfile

After installing VirtualBox (https://www.virtualbox.org/) and Vagrant
(https://www.vagrantup.com/), developers can use

	vagrant up

to obtain an Ubuntu development environment that can build Fiji --
regardless of the current host operating system.

This development environment is a dedicated, headless VirtualBox
instance into which -- once started up via 'vagrant up' -- the developer
can log in vi 'vagrant ssh'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>