memory_backend config options should be scoped under {riak_kv, [...]} not as a top level app

Current docs instruct to configure memory_backend as a top level application in app.config (`{memory_backend,[ ... ]}`) .  Any options configured in this manner are ignored as riak_kv looks for them in the `{riak_kv, [ ... ]}` proplist.  This applies to the `max_memory`, `ttl`, and `test` options in the memory_backend.