Implement plugin management solution

https://github.com/tknerr/vagrant-plugin-bundler

This seems to be a partial solution for ensuring that everyone has the correct plugin versions, at least until more full-fledged bundler support lands.

EDIT: A better solution has landed with [Vundler](https://github.com/fgrehm/vundler).

The one downside is that it seems to make the setup VERY opinionated. I'd been working under the assumption earlier that while we could support the use of helpful plugins, it should be possible for someone to choose not to use any or all plugins, without having the hack apart our code:

https://github.com/patcon/vagrant-prototyping/blob/develop/Vagrantfile#L20
https://github.com/patcon/vagrant-prototyping/blob/develop/lib/proviso/helpers.rb#L11-L14

The downside was that this helper function approach created a more verbose `vagrantfile`. An alternative might be to submit something to vagrant core so that a single config flag can be set that turns all those `unknown config option` errors (usually symptomatic of a vagrant plugin being missing) into warning that don't fail the `vagrant up`. That might be a better approach

Feel free to drop in thoughts. It's a little tangential, but this seems as good a place as any to talk it through :)