openapi-spec: support the upcoming 3.0 flavour of OpenAPI Spec

[bajtos]

This is a follow-up issue for #177, it may be a duplicate of #753

[bajtos]

Quoting from https://www.openapis.org/blog/2017/05/25/the-open-api-initiative-is-sending-you-a-save-the-date-card:

At its meeting on Friday, 19 May, the Technical Developer Community (TDC) agreed to target the below timeline for release:

• 4 weeks from 22 May through 16 June: Resolve remaining pull requests
• 2 weeks from 19 June through 30 June: Comment period
• 2 weeks from 3 July through 14 July: Resolve remaining comments and issues
• 1 week from 17 July through 21 July: TENTATIVE release window