MB-8191: Logo update.

Change-Id: I0cf38845ddf62ee06268a9f1ab837822b217edb4
Reviewed-on: http://review.couchbase.org/26212
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>