bump nats [delivers #79543220]

- Go back to apcera/nats from our fork now that client ping has been
merged in.

Submodule src/github.com/apcera/nats f4b6116...e17cb62:
  > Merge pull request #39 from fraenkel/client_ping
  > Minor cleanup, version bump
  > Merge pull request #36 from fraenkel/reconnect
  > Merge pull request #37 from apcera/fix-goveralls
  < Merge remote-tracking branch 'upstream/master' into reconnect-on-stale
  < NATS client should timeout on Reads eventually [#81327762]
  < Treat stale connections as temporary/recoverable errors

Signed-off-by: Amit Gupta <agupta@pivotal.io>
Use cats-ginkgo-opts stub in run_nondiego_cats script to generate manifest

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>