bp:MB-5307: use -Wno_improper_lists dialyzer options

I.e. because we had to forker file_sorter from erlang that is not
dialyzer-clean without that option.

Orig-Reviewed-on: http://review.couchbase.org/23594

NOTE: we need to backport it, even if it's MB-5307, to keep ns_server
dialyzer clean.

Change-Id: Ifaaef60254ac751dd7cff1f2a179a0ec567ead48
Reviewed-on: http://review.couchbase.org/23670
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>