Validate that jwks and jwks_uri are not used together

[adalinesimonian]

Fixes #98

This involved customizing the validation function for the Client model because there is no logic in Modinha or Revalidator that supports cross-property validation rules.

cc @christiansmith

[christiansmith]

@vsimonian that's a really good point. I think we should either contribute to revalidator to add multi-property validation, add some layer into Modinha that adds this behavior by wrapping revalidator, or write our own JSON Schema validator entirely within Modinha that incorporates this and other features we've bolted on.

[adalinesimonian]

We might consider looking at lx-valid - they've extended revalidator with useful utilities we may take advantage of, all while maintaining backwards-compatibility. They also seem to be in active development, whereas Revalidator itself seems a bit stale.

[christiansmith]

This PR looks good. Merging. Thanks.