Revert the removal of buildah dependency

We currently need it to drag in recent versions of other dependencies,
per https://github.com/containers/skopeo/issues/796 .

I'll work to update the relevant dependencies in c/image, but that will
only propagate to skopeo in the next c/image release; in the meantime,
this at least undoes the downgrades.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/github.com/docker/go-metrics/unit.go

@@ -0,0 +1,12 @@
+package metrics
+
+// Unit represents the type or precision of a metric that is appended to
+// the metrics fully qualified name
+type Unit string
+
+const (
+	Nanoseconds Unit = "nanoseconds"
+	Seconds     Unit = "seconds"
+	Bytes       Unit = "bytes"
+	Total       Unit = "total"
+)