Update deployments-balancer example to use Ubuntu 18.04.

Author: geerlingguy

deployments-balancer/Vagrantfile

@@ -3,7 +3,7 @@
 
 Vagrant.configure("2") do |config|
  # Base VM OS configuration.
- config.vm.box = "geerlingguy/ubuntu1404"
+ config.vm.box = "geerlingguy/ubuntu1804"
  config.vm.synced_folder '.', '/vagrant', disabled: true
  config.ssh.insert_key = false