By FIONA ROBERTS Whoa! Six-year-old Marcus Paul clings on to his sheep with steely determination as he competes in Mutton Bustin' at the Houston RodeoIf putting your five-year-old on a 150lb sheep with only their determination to keep them on doesn't sound like the wisest of ideas, you shouldn't go to Texas.Every night for the last three weeks, 15 tiny sheep riders have been competing in front of