Replace Poison with Jason

[ybod]

1. Replace Poison with Jason for JSON encoding/decoding
2. Update hex_doc version
3. Format code with mix format

[ntrepid8]

What is the motivation for using Jason rather than Poison?

[ybod]

Jason is faster and "smaller", many projects already switched to Jason.

[cgorshing]

I came here to request these changes as well. As Phoenix comes with Jason already as a dependency, many projects are following suite to keep from bringing in dependencies that do the same thing.

If you don't want to use a json library directly, then maybe we could tie it to a behaviour or something like https://github.com/scrogson/oauth2/#configure-a-serializer

Another example of people asking for this
parroty/extwitter#121

[ntrepid8]

I'm not opposed to using Jason :)

Mostly I need to figure out why Travis isn't running the CI jobs anymore :(

[barttenbrinke]

Can somebody merge this? Oh, I see Poison has a new maintainer now 🤔