Set release date of 3.6.8 to today

Set release date of 3.6.7 to today

Update version to 3.6.6

Set release date of 3.6.5 to today

Update Gemfile.lock for nanoc 3.6.4

Set 3.6.3 release date to today

Set release date of 3.6.2 to today

Update release notes for 3.6.1

Fix crash in stale check when prune config is incomplete