Update to Clair V3 API Breaking Build

[rikkuness]

Seems that an upstream change to the Clair V3 API is going to break builds in future.
quay/clair@5d725e6#diff-df6af5631723ec819c96e4a947bfd5ba

Affects:
https://github.com/genuinetools/reg/blob/master/clair/ancestry.go#L18-L19

I'll update this issue once I find out more.

[rikkuness]

PR #140 should fix, all tests pass but I don't currently have a live test environment to hand to test it against.