CUDA version changes when upgrading Cog #330

If you upgrade Cog and the latest known CUDA version changes in Cog, then Cog will start using a more recent version of CUDA for your version.

As a user, I would expect my Cog environment to remain consistent. This is unexpected behavior, and might cause breakage.

A couple of potential solutions:

1. We could encourage users to specify a cuda version.
2. We could lock the CUDA version somehow, akin to package-lock.json.

See #329 (comment)