Change gitlab/log permissions in installation.md

This patch changes default permission of the gitlab/log directory to
u+rwX,go-w. This is done to make the directory NOT readable by group and
others and to avoid logrotate complaining about it. chmod 755 is not
used to avoid setting executable bit on file within the log dir.
Merge pull request #7652 from m-a-r-c-o/logrotate-error-bugfix

Fix log rotate permission error