Update protagonist to AST serialization JSON media type v2.0

**please read carefully** 

This is an API breaking change. As of this PR protagonist will 1:1 conform to AST serialization JSON media type v2.0. Please refer to the AST serialization [v2.0 release](https://github.com/apiaryio/api-blueprint-ast/releases/tag/v2.0) for details on braking changes.