Decode NS, BS, SS to MapSet by default, switch :dev and :test env JSON codec to Jason

In this major version release, we now support decoding Dynamo's number set, binary set, and string set types to Elixir MapSet as the default behavior (previously, these would decode to List types, unless configured otherwise).

Furthermore, we've swapped out the JSON codec used for :dev and :test environments from Poison to Jason, in keeping with the default behavior of the ex_aws project. The docs also advise using Jason instead of Poison in your app.

For a slightly more detailed summary, please see the upgrade guide.