Add a virtual destructor for the Notifiable class

Classes with virtual functions should also have a virtual
destructor.

Change-Id: I1dfc948000f51d6ac61ad86b673410ee15cedeca
Reviewed-on: http://review.couchbase.org/37862
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>