rafka 0.4.0

This is a maintenance release adding support for librdkafka 0.11.6 and beyond.

Fixed

• With librdkafka 0.11.6 and 1.0.0, consumers could block indefinitely in BLPOP,
  resulting in rafka being unable to shutdown [#77]

Added

• "Unsupported command" errors now contain the actual command that was
  attempted by the client [fa45217]

Changed

• If there are no messages to be consumed, BLPOP returns a "null array" instead
  of a "null string", adhering to the Redis protocol. This is an internal change
  that shouldn't affect clients [adf3650]

• Connection write/flush errors are not logged anymore [686af22]