* We were trying to delete Job IDs which didn't exist in Kubernetes.
  To fix this a new method has been added to resolve the REANA IDs
  to backend IDs, which will be helpful once we implement stop
  for HTCondor jobs (closes reanahub/reana-client#314).

* WorkflowRunManger can determine which jobs to delete by itself.

* pytest-reana declares as dependency `mock>=2` and Travis installs
  2.0 instead of lasted one (3.x). To be removed once we upgrade the
  `mock` version in pytest-reana.