text: Title:  __init__() missing 2 required positional arguments: 'schema_name_resolver' and 'spec'
  Body: Hello, I'm getting the error below. Am I missing anything?
  
  ```
  ../../../env36/lib64/python3.6/site-packages/flask_base/app.py:1: in <module>
      from flasgger import Swagger, LazyString, LazyJSONEncoder
  ../../../env36/lib64/python3.6/site-packages/flasgger/__init__.py:8: in <module>
      from .base import Swagger, Flasgger, NO_SANITIZER, BR_SANITIZER, MK_SANITIZER, LazyJSONEncoder  # noqa
  ../../../env36/lib64/python3.6/site-packages/flasgger/base.py:37: in <module>
      from .utils import extract_definitions
  ../../../env36/lib64/python3.6/site-packages/flasgger/utils.py:22: in <module>
      from .marshmallow_apispec import SwaggerView
  ../../../env36/lib64/python3.6/site-packages/flasgger/marshmallow_apispec.py:13: in <module>
      openapi_converter = openapi.OpenAPIConverter(openapi_version='2.0')
  E   TypeError: __init__() missing 2 required positional arguments: 'schema_name_resolver' and 'spec'
  ```