Fix for: + '[' -z Acquire::http::Proxy '"http://proxy.example.com:80";' ...

Trying to run `vbox_create.sh` behind a proxy does not work today as the Vagrant VM hangs trying to do the `apt-get update` & `apt-get dist-upgrade` [dance](https://github.com/bloomberg/chef-bcpc/blob/master/bootstrap_chef.sh#L77).

This fix has been in the Hadoop branch since December 30th successfully and would help folks build chef-bcpc behind a proxy using [`test/automated_install.sh`](https://github.com/bloomberg/chef-bcpc/blob/master/tests/automated_install.sh).