Delete old .deb files to save disk space

Change-Id: I7a21a2638c67b7f9576ff0f8ccd2964e1283e909
Reviewed-on: http://review.couchbase.org/44262
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>