Prevent parsing and returning of spec-definied models

Part of the swagger API specification is the ability to define data structures ('models') for API responses. Recent updated to bravado-core (~4.13) and the swagger spec published by CCP have resulted in parsing of spec models and two distinct errors:

• duplicate model names are defined in the spec provided by CCP which raises an error on swagger spec parsing
• returned models are not pickle-able which breaks response caching

This release adds an extra kwarg to spec generation which prevents returning data as models, favouring dictionaries.