vendor: Bump to include containers/image

And zounds of other dependencies (Prometheus!).  We aren't actually
using most of these, but dep does not currently support pruning by
build tag [1], and a few of the packages slop some unrelated stuff
together.  For example, Prometheus comes in via:

  github.com/containers/image/docker
    github.com/docker/distribution/registry/client
      github.com/docker/distribution/registry/storage/cache
        github.com/docker/distribution/metrics
          github.com/docker/go-metrics
            github.com/prometheus/client_golang/prometheus

and BoltDB comes in via:

  github.com/containers/image/pkg/blobinfocache
    github.com/boltdb/bolt

we don't use either the storage backend or the BoltDB blob-info cache,
but dep isn't checking at that level of granularity.  Ideally the
upstream repositories would restructure to split these out into
separate packages, but until then, just commit all the cruft dep
brings in.

Generated with:

  $ dep ensure

using:

  $ dep version
  dep:
   version     : v0.5.0-31-g73b3afe
   build date  : 2019-02-08
   git hash    : 73b3afe
   go version  : go1.10.3
   go compiler : gc
   platform    : linux/amd64
   features    : ImportDuringSolve=false

[1]: golang/dep#291