Vagrantfile: add a configuration patch to fix slow networking on some hosts.

See also https://github.com/mitchellh/vagrant/issues/1172 and https://gist.github.com/simono/73e37d8c3a45664c7045 for a description of the problem and the origins of this fix.

Signed-off-by: Eric Myhre <eric@flynn.io>
Merge pull request #589 from flynn/patch-vagrant-networking-config

Vagrantfile: add a configuration patch to fix slow networking on some hosts.