vagrant: use ubuntu 16.04 LTS

Vagrantfile

@@ -7,7 +7,7 @@ Vagrant.configure(2) do |config|
 
   # Every Vagrant development environment requires a box. You can search for
   # boxes at https://atlas.hashicorp.com/search.
-  config.vm.box = "boxcutter/ubuntu1510"
+  config.vm.box = "boxcutter/ubuntu1604"
 
   # Disable automatic box update checking. If you disable this, then
   # boxes will only be checked for updates when the user runs