[skip ci] Remove reference to outdated python-memcached version
[skip ci] Remove reference to outdated python-memcached version

https://github.com/linsomniac/python-memcached/ 1.5.8
had the delete_multi signature fixed in 2016.
Anyone who had installed an outdated version should have
seen this recommendation already or worked around it differently.

There are a large number of other languages that have their own memcache
clients with different bugs, don't continue linking to the workaround
for an outdated python-memcached client version

Closes #475

andyqzb/twemproxy,twitter/twemproxy,andyqzb/twemproxy,andyqzb/twemproxy,twitter/twemproxy,andyqzb/twemproxy,twitter/twemproxy,twitter/twemproxy