Better member join date ordering in bp_group_has_membrs_with_exclude_banned_0()

This ensures that the order of returned results will be correct on slower test
rigs like Travis. It also accounts correctly for the date_modified date of the
group creator.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7330 cdf35c40-ae34-48e0-9cc9-0c9da1808c22