use --rebase rather than --skip-if-exists

other occurrence is fine; it's uploading an exact .tgz

[#81029558]
bump converger inigo runtime-schema

Submodule src/github.com/cloudfoundry-incubator/converger bbc9118..b2b56d1:
  > Thread expire completed task duration up to main
Submodule src/github.com/cloudfoundry-incubator/inigo 4db6cf6..afb5bd3:
  > Unresolved tasks stuck in Resolving or Completed get deleted
Submodule src/github.com/cloudfoundry-incubator/runtime-schema 3e0847c..7772048:
  > Delete tasks that are in 'Completed' for too long

Signed-off-by: Nick Wade <nwade@pivotal.io>
Create final release 0.491 for build 245