memcached: bump to version 1.5.6

memcached: bump to version 1.5.6

From ReleaseNotes156:

"""
This is a bugfix release, but it primarily disables the UDP protocol by
default.

In the last few days reports of UDP amplification attacks utilizing
inesure memcached instances have surfaced. Attackers are able to set
large values into memcached, then send requests via spoofed UDP packets.
Memcached will then send a very large number of very large UDP packets
back in response.
"""

Signed-off-by: Christopher McCrory <chrismcc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>