ENH: "Better" descriptive string for time diff

When the time difference is only slightly negative,
use "just now" as a descriptive string for when the
build occurred. "Some time in the future" is a little
freaky and unnerving when the only thing wrong is a
slight clock discrepancy on the CDash server.

git-svn-id: https://www.kitware.com/svn/CDash/trunk@3338 91079597-3006-4846-9bd3-4f3fb3d139a7