Merge tag '6.1.beta6' into t/15606/ticket/15606

Sage version 6.1.beta6
bump confball version
Trac #15606: Move checks from prereq.sh to configure

Follow-up after #15580.

URL: http://trac.sagemath.org/15606
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun