NPE when responses section is missing from an operation

Issue #130 resolved
Former user created an issue

Missing responses is not valid spec according to https://github.com/OAI/OpenAPI-Specification/blob/master/schemas/v2.0/schema.json#L241 but it would be good for swagger-request-validator to throw an assertion error, or at least an expressive exception, rather than an NPE in this case.