Wrong ApiSpec class used

[dmitrymishanov]

Here ApiSpec class from apispec is used, but here register_field method of combojsonapi.spec.apispec.APISpec is called (and register converter below too).
Seems like it doesn't make any problems now because _fields and _converters attributes of ApiSpecPlugin are always empty lists.

We should either 1) remove combojsonapi.spec.apispec.APISpec, usages of its methods and _fields and _converters attributes of ApiSpecPlugin or 2) change ApiSpec class in ApiSpecPlugin.__init__ if there are some reasons for its existence.