We don't usually run "acceptance tests" during a Travis run, but this
particular suite doesn't require any special credentials since it just
accesses releases.hashicorp.com to download plugins, so therefore it's
safe to run in Travis at the expense of adding a few more seconds to
the runtime.

Running it in Travis can therefore give us some extra confidence for
pull requests that may inadvertently break certain details of the
workflow, as well as ensuring that these tests are kept up-to-date as
the system changes.