rand_core 0.6.4 is not published

[Kixunil]

The version was bumped in git a year ago but never published.

[nathanfranke]

Yeah, I think this is valid, can't use CryptoRngCore.

[dhardy]

You're right, this should have been published over a year ago. Since then a couple more changes have landed, but nothing breaking, so I updated the changelog.

[Kixunil]

Thanks!