!act #3526 Make SupervisorStrategy.logFailure public

* This change is not source compatible since anyone that has
  overridden logFailure will have compilation error
  "has weaker access privileges; it should be public"
Merge pull request #1648 from akka/wip-3526-public-log-failure-patriknw

!act #3526 Make SupervisorStrategy.logFailure public