Bump src/cloud_controller_ng

Bump cloudfoundry/cloud_controller_ng:
  Matthew Sykes:
     Avoid nil start commands when droplet is missing ( Travis Success: https://travis-ci.org/cloudfoundry/cloud_controller_ng/builds/31766038 )
     Add `docker_image` to app table, model, controller ( Travis Unknown )
     Remove unused method ( Travis Unknown )
     Move dea advertisement subscription to stager_pool ( Travis Unknown )

Signed-off-by: Peter Parente <pparent@us.ibm.com>