WIP: Version 5

[benarent]

I've created this issue to explore creating a new breaking version of this library.

Todos:

• The V3 JSON API to create errors https://airbrake.io/docs/#create-notice-v3 ( remove XML support )
• Only support Rails 4.x
• Make async-only

Things we'll need to consider.

• The V3 JSON API requires an API Key and Project ID.
• Need to update docs.
• An option of including github.com/airbrake/airbrake-js as using standard best practices in Rails 4.
• Remove MultiJson and use JSON