Nudging users off 2015 edition

[mathstuf]

@edwloef

"a mostly mechanical change" and it "would pollute the Git history (especially the blame view) with churn". Breaking such code would be very unfortunate in my opinion.

The blame.ignoreRevsFile configuration exists which can cause commits to be skipped when blaming. Prior to learning about this, we use --author="Robot <robot@meail.address>" to indicate such things.

[ekuber]

This will be merged soon.

The only thing that we might want to change is that it currently eprintlns a message if --edition is unset, but we might want to emit it only if errors are present?