Update from ubuntu1404 to ubuntu1804 in dynamic-inventory example.

Author: geerlingguy

dynamic-inventory/custom/Vagrantfile

@@ -10,14 +10,14 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   # Application server 1.
   config.vm.define "inventory1" do |inventory|
     inventory.vm.hostname = "inventory1.test"
-    inventory.vm.box = "geerlingguy/ubuntu1404"
+    inventory.vm.box = "geerlingguy/ubuntu1804"
     inventory.vm.network :private_network, ip: "192.168.28.71"
   end
 
   # Application server 2.
   config.vm.define "inventory2" do |inventory|
     inventory.vm.hostname = "inventory2.test"
-    inventory.vm.box = "geerlingguy/ubuntu1404"
+    inventory.vm.box = "geerlingguy/ubuntu1804"
     inventory.vm.network :private_network, ip: "192.168.28.72"
   end
 end