Consider changing to Jason json library

[nacengineer]

The poison library has quite a few open issues. Including at least one for coming Elixir deprecations, e.g. use Bitwise, and devinus has been silent on adopting it/them. Not saying poison is dead but 2 years without pulling in easy PRs is kind of that to me.

Consider pivoting to the more maintained Jason library.

[jdfrens]

There's an unadvertised config option: :json_encoder.

config :airbrake_config, 
   ...
  json_encoder: Jason,
  ....

I'm reluctant to make Jason the default because it'll blow up on structs that don't have encoders (where I think Poison just encodes them as maps).