By Ginger Gibson and Alana Wise INDIANAPOLIS, Ind. (Reuters) - Billionaire Donald Trump assumed the mantle of presumptive Republican presidential nominee on Wednesday with a message on unity that also suggested he was not going to work too hard to placate some party establishment figures angered by his outsider candidacy. NBC News reported on Wednesday that Trump's last remaining challenger, Ohio Governor John Kasich, was suspending his campaign after his dismal third place finish in the Midwestern state. There was no immediate word on this from Kasich's campaign, but he canceled a news conference in the Washington area and planned to make a statement at 5 p.m. ET in Columbus, Ohio.