Update Vagrant box to Bionic (#13384)

Vagrantfile

@@ -91,7 +91,7 @@ VAGRANTFILE_API_VERSION = "2"
 
 Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
 
-  config.vm.box = "ubuntu/xenial64"
+  config.vm.box = "ubuntu/bionic64"
 
   config.vm.provider :virtualbox do |vb|
     vb.name = "mastodon"