Pass an external_encoding: option to IO.popen, to avoid an exception.

This only works for Ruby 1.9+, but hey, that's the future.
Nix all references to VirtualBox ...

... in an attempt to get

   /usr/local/Cellar/ruby/2.0.0-p195/lib/ruby/gems/2.0.0/gems/bundler-1.3.5/bin/bundle exec veewee fusion build 'ubuntu-small-disk'

to stop trying to install the virtualbox guest tools