bump volman
bump volman

Submodule src/github.com/cloudfoundry-incubator/volman af06b23..867cf31:
  > add reference count logic to fakedriver so that we only unmount volumes when the last container is done with them. [#120382681](https://www.pivotaltracker.com/story/show/120382681)

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release