Make logic consistent with what we're using elsewhere.

Without this change, e.g. 0.5.20 would not get --extra-clean, while it should.
Granted, the new test is buggy too, e.g. 0.7.1 will get --extra-clean,
while it should not. Will report that to Redmine.

handlers/dup.in

@@ -191,7 +191,7 @@ else
 fi
 
 ### Cleanup options
-if [ "$duplicity_major" -le 0 -a "$duplicity_minor" -le 6 -a "$duplicity_sub" -le 19 ]; then
+if ! [ "$duplicity_major" -ge 0 -a "$duplicity_minor" -ge 6 -a "$duplicity_sub" -ge 20 ]; then
    execstr_options="${execstr_options} --extra-clean"
 fi