(Adds details, quotes) By Julien Pretot BOURG-SAINT-ANDEOL, France, July 15 (Reuters) - The Tour de France resumed under heightened security and in a low-key mood on Friday after race officials observed a minute's silence in tribute to the scores of victims of the Bastille Day attack in Nice. A gunman at the wheel of a heavy truck ploughed into a crowd celebrating France's national day in the southern city late on Thursday, killing at least 84 people and injuring scores in what President Francois Hollande called a terrorist act. The stage is on," said race director Christian Prudhomme. More...