Fix for getServerByKey() bug (Github Issue #70)

Fix for Github Issue #70 where getServerByKey() returns bad values for recent libmemcached versions.  See: https://github.com/php-memcached-dev/php-memcached/issues/70

Appears to be related to a structure change inside libmemcached, as shown in Launchpad Bug #1076048.  See: https://bugs.launchpad.net/libmemcached/+bug/1076048