Invalid specs are accepted from swagger spec validator (operation responses cannot contain references)

[macisamuele]

While working on Yelp/bravado-core#263, @sjaensch noticed that the produces swagger specs were not valid :(
This was caused by the fact that we started from invalid swagger specs and that swagger_spec_validator didn't spot the validation issue.

According to swagger 2.0 specs responses object should not contain references.
Allowed attributes are "default", http status codes, x-vendor attributes.

How to reproduce the described issue

In [1]: import sys

In [2]: sys.platform
Out[2]: 'darwin'

In [3]: sys.version_info
Out[3]: sys.version_info(major=3, minor=6, micro=5, releaselevel='final', serial=0)

In [4]: from swagger_spec_validator.validator20 import validate_json

In [5]: spec_dict = {
   ...:   "swagger": "2.0",
   ...:   "info": {
   ...:     "title": "Test",
   ...:     "version": "1.0"
   ...:   },
   ...:   "responses": {
   ...:     "default": {
   ...:         "description": "any response"
   ...:     }
   ...:   },
   ...:   "paths": {
   ...:     "/endpoint": {
   ...:         "get": {
   ...:             "responses": {
   ...:                 "$ref": "#/responses"
   ...:             }
   ...:         }
   ...:     }
   ...:   }
   ...: }

In [6]: validate_json(spec_dict, 'schemas/v2.0/schema.json')
Out[6]: <jsonschema.validators.RefResolver at 0x10bcde128>

In [7]:
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