.travis.yml: Do not log all dates
.travis.yml: Do not log all dates

We check dates in set.mm using scripts/report-changes.py
(to ensure each date exists).  This commit sends the date information
to /dev/null, because otherwise the log file is too long.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm