Update Vagrantfile to ubuntu/xenial64.

The version of Redis that ships with Ubuntu Trusty is too old to support
the HyperLogLog commands.  I guess we could add a custom apt repo for
this, but it seems a lot easier to just use the latest Ubuntu LTS
release (i.e. xenial), which has a recent-enough Redis in its archives.

Vagrantfile

@@ -79,7 +79,7 @@ VAGRANTFILE_API_VERSION = "2"
 
 Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
 
-  config.vm.box = "ubuntu/trusty64"
+  config.vm.box = "ubuntu/xenial64"
 
   config.vm.provider :virtualbox do |vb|
     vb.name = "mastodon"