fixes #479 - Recover running deployments on failover or restart
Tweaked task ID mocks in DeploymentActorTest

- StartingBehavior.runningTasks was an Int, is now a Set.
  This caused breakage when the mock emitted duplicate IDs
Merge pull request #605 from mesosphere/wip-fault-tolerant-deployments-drexin

fixes #479 - Recover running deployments on failover or restart