By Marice Richter DALLAS (Reuters) – Grammy award-winning country music singer Randy Travis is responding to treatment for a heart condition that has kept him hospitalized in critical condition since Sunday, his doctors said on Wednesday. The cause of his illness was a virus, according to Dr. William Gray, director of cardiovascular services at Baylor Medical Center at McKinney, about 30 miles north of Dallas. Travis' doctors spoke publicly for the first time Wednesday in a video news conference but did not give details about his treatment. …