missing dist in cpanfile uses artifact cache by default

[miyagawa]

Fallout from #64

When you add a new module to cpanfile and run carmel install it will try to use a version from the artifact repository, if it found one that's matching. This speeds up things but introduces an inconsistent behavior, based on user's local environment.

It should only use the version in the cache if that's also the latest version on CPAN, like now Carmel does on the first run of carmel install without the snapshot.