HPCC-12728 The memcached plugin does not compile on Ubuntu 12.04

Because the version of libmemcached headers is too old.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6744 from richardkchapman/libmemcached-old-version

HPCC-12728 The memcached plugin does not compile on Ubuntu 12.04

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>