Fix unicorn/nginx 403 auth errors

This addresses issue https://github.com/puppetlabs-operations/puppet-puppet/issues/119 by replacing the location that the jfryman/nginx vhost creates by default with an explicitly defined one, because the default one [(indirectly) sets `Host`, `X-Real-IP` and `X-Forwarded-For` headers ](https://github.com/jfryman/puppet-nginx/blob/v0.0.10/manifests/params.pp#L46) that we define outside of the location block. 

I don't understand why setting those headers there is a problem, but I tested simply removing those three headers from the server block and that didn't fix the issue; I still got 403 errors when the agent connected.

This additionally updates the Vagrantfile. I keep making the same set of changes, so figured I'd wrap it in. I realize it doesn't quite fit the rest of the PR but I don't think it should offend anybody. 

Finally, the rack config is updated to allow for puppet 3.7. That's also not related to this issue, just a needed and harmless fix.