Require vagrant-vbguest for managing virtualbox guest additions

This is a fix for https://github.com/litaio/development-environment/issues/1

I found Vagrant.require_plugin over on [this stackoverflow thread](http://stackoverflow.com/questions/19492738/demand-a-vagrant-plugin-within-the-vagrantfile), but that's been deprecated and has no effect. I found an alternative over on https://github.com/mitchellh/vagrant/issues/4349 using this: https://github.com/rapidftr/RapidFTR/blob/master/Vagrantfile#L7

Basically copied that, and update it to vagrant-vbguest.

Not sure if it's something in my local environment, but I hit something like https://github.com/mitchellh/vagrant/issues/3769 when installing the plugin, but I think that may be more my environment. I was able to work around that with `export NOKOGIRI_USE_SYSTEM_LIBRARIES=true`.