Increase timeout by 50% in thrashers/mapgap.yaml

Seen in /a/teuthology-2013-12-17_23:00:03-rados-next-distro-basic-plana/7188

Signed-off-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit 20b7998222c51f6c0aeab618c0f3cf8e85a90e40)