Merge branch 'fix-async-producer2' into 'master'

add external state for async_producer so that it survives the crash

See merge request !15

Merge branch 'fix-async-producer' into 'master'

fix async producer

See merge request !14

Update rebar3_elixir for Elixir 1.4

Update dependencies graph

Bump kafe to 2.1.6

Prepare 0.6.2

Bug correction in mix.exs

Bump dependencies

Bump kafe to 2.1.2

Update wok_message_handler

Prepare 0.5.0