Set single class for stale/good

- Before this was adding a class each time, so colours would fail to update
- Now just sets a single class, which updates the colour correctly
Remove oldest commit time if doesn't exist
Merge pull request #3 from evilstreak/fix-stale-display-bugs

Fix stale display bugs
Retitle oldest commit as oldest merge

Commit e3bef0d updated the timestamp to show the age of the oldest merge
rather than the oldest commit. This updates the title to match.
Merge pull request #4 from evilstreak/update-oldest-commit-title

Retitle oldest commit as oldest merge