Adapted to Elixir 0.13.0
Replacing Record PubAckMsg by a Struct
Preparing for Elixir 0.13.1 and more structs everywhere
Using Maps directly (for Elixir 0.13.1)
With maps and that stuff...
Doc: QoS2 is Exactly Once
New module for defining message structs and convenience functions
Travis goes for OTP 17.0 and Elixir 0.13.2