vagrant: use xenial64 instead of vivid64
vagrant: use xenial64 instead of vivid64

The vivid64 image is no longer available.

jonboulle/acbuild,joshix/acbuild,jonboulle/acbuild,joshix/acbuild,containers/build,appc/acbuild,dgonyeo/acbuild,dgonyeo/acbuild,appc/acbuild,containers/build