By Ruby Tevis Portraits by Adrienne Morella When Becca Roth thinks about her future, she envisions galloping along the cross-country course at the Olympic Games. The 17-year-old eventer from Fond Du Lac, Wisconsin, has big dreams and the courage to conquer them. With her partner, an off-the-track-Thoroughbred called Deuce, Becca has already achieved a lifetime’s