contrib: bump msgpack

Bump msgpack to latest stable version 2.1.5
Relevent fixes for Ring:
* fix unpack visitor to treat float32/64 correctly
* fix alignment problem on some platform (includes arm)

Change-Id: I3c3300c02e91435b0e72ae48e2614efad3e3b936
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>

contrib/src/msgpack/rules.mak

 # MSGPACK
-MSGPACK_VERSION := cpp-2.1.3
+MSGPACK_VERSION := cpp-2.1.5
 MSGPACK_URL := https://github.com/msgpack/msgpack-c/archive/$(MSGPACK_VERSION).tar.gz
 
 PKGS += msgpack