Ignore "required" field on grouped parameters

The [Swagger spec](https://github.com/wordnik/swagger-spec/blob/master/versions/1.2.md#52-api-declaration) doesn't have any support for grouped parameters. They're a [Grape thing](https://github.com/intridea/grape#parameter-validation-and-coercion).

Currently `grape-swagger` is ignoring the description of parameter groups, and just trusting the `requires` or `optional` state of the contained leaf parameters. This leads to trouble when using something like [swagger-ui](https://github.com/wordnik/swagger-ui) for documentation and sandboxing, as it may require a user to enter certain parameters that shouldn't actually be required.

The hacky fix here is to ignore the `required` bit on parameters that are part of a group. It doesn't really fix the problem, it lessens the impact.