bucket_engine is now part of memcached

Change-Id: I9bef17d350b0d1cbc7a7873cf1ac14677a488047
Reviewed-on: http://review.couchbase.org/25376
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>