Bump version of memtier submodule

memcached uses memtier benchmark v. 1.2.6 (Sept 2015).
There are many changes were added since 1.2.6: https://github.com/RedisLabs/memtier_benchmark/compare/40d47a0e3b84b14e2ca88d9f0d506b4f04e39b16..master
and it would be good to bump it's version in memcached.