Only update USD value to unconfirmed bounties

Fixes: #148

resources/sql/queries.sql

@@ -493,7 +493,7 @@ SELECT
 FROM issues i, repositories r
 WHERE r.repo_id = i.repo_id
 AND contract_address IS NOT NULL
-AND i.payout_hash IS NULL;
+AND i.confirm_hash IS NULL;
 
 -- :name get-bounty :? :1
 -- :doc details for a bounty issue given owner, repo and issue nunber