* ECB’s governing council to hold call this afternoon * Aetna falls after Humana deal * All 30 Dow components in the red * Energy index leads losses as oil prices fall * Indexes down: Dow 0.72 pct, S&P 0.67 pct, Nasdaq 0.69 pct (Updates to open) By Tanya Agrawal July 6 (Reuters) – U.S….