WASHINGTON (Reuters) - U.S. House of Representatives Speaker Paul Ryan on Thursday rejected a warning by Donald Trump of riots if the businessman does not win the Republican presidential nomination for the Nov. 8 election. Asked about Republican front-runner Trump’s warning, Ryan told reporters that “nobody should say such things.” He added: “To even address or hint to violence is unacceptable.” Ryan, who will oversee Republicans’ convention in July to pick the party’s presidential candidate, said he does not think he will have to denounce Trump as a candidate.   