pgmemcache: correct free function for memcached_stat_get_value

libmemcached 1.0.16 and greater switched to using malloc for
values returned from memcached_stat_get_value.
Merge branch 'master' of git://github.com/melor/pgmemcache

* 'master' of git://github.com/melor/pgmemcache:
  pgmemcache: correct free function for memcached_stat_get_value