bump nats

Submodule src/github.com/apcera/nats fdac636...7b5adb1:
  > Treat stale connections as temporary/recoverable errors
  < Print the err when processErr is called [#76625036]
  < Add stack trace when nats client is closed

Signed-off-by: Abhijit Hiremagalur <abhi@pivotal.io>