Enable subdomain multisite installations

Dropped dependency on `vagrant-hostsupdater` in favor of [`landrush`](https://github.com/phinze/landrush). `landrush` spins up a small dns server that allows us to use wildcard subdomains, a requirement for subdomain multisite installs. Let me know what you think!
How about:

```ruby
PRIVATE_IP = '192.168.50.5'
...
config.vm.network :private_network, ip: PRIVATE_IP
...

if Vagrant.has_plugin? 'landrush'
  config.landrush.enabled = true
  config.landrush.tld = config.vm.hostname

  host_aliases = other_sites.flat_map { |site| site['site_hosts'] }
  host_aliases = (host_aliases - [config.vm.hostname]).reject { |hostname| hostname.starts_with? '*' }

  host_aliases.each do |hostname|
    config.landrush.host hostname, PRIVATE_IP
  end
else
...
end
```

This way we're re-using the private IP and `config.vm.hostname`.