mosquitto: security bump to version 1.4.15

mosquitto: security bump to version 1.4.15

Fixes CVE-2017-7651: Unauthenticated clients can send a crafted CONNECT
packet which causes large amounts of memory use in the broker.  If multiple
clients do this, an out of memory situation can occur and the system may
become unresponsive or the broker will be killed by the operating system.

The fix addresses the problem by limiting the permissible size for CONNECT
packet, and by adding a memory_limit configuration option that allows the
broker to self limit the amount of memory it uses.

The hash of new tarball is not (yet) available through download.php, so use
a locally calculated hash.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>