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>
Merge branch 'vagrant'

This makes it easy to obtain an Ubuntu Linux virtual machine by running
'vagrant up' followed by 'vagrant ssh'. It only requires VirtualBox and
Vagrant to be installed.

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