Bump ring dependency to 0.17.0

[grubles]

Hello. I'd like to run some rust code that uses rcgen as a dependency on a powerpc64le machine. Recently, ring added support for this architecture in version 0.17.0.

Is there anything preventing using 0.17.0 instead of 0.16.20?

[iamjpotts]

There's an active PR for it: #166

[grubles]

Ok thanks. I'll close this then.