vagrantfile: use xenial throughout
vagrantfile: use xenial throughout

As of SecureDrop 0.12.0, Xenial is the default environment for
SecureDrop servers. Trusty will soon be deprecated.

conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop