By SARA VANDEN BERGE Round up a chair, cooler of drinks and your favorite people and make plans to head to Stephenville City Park this weekend to catch Ricky Skaggs and Kentucky Thunder. The performance is part of the annual summer concert series held at Birdsong Amphitheater. It will begin at 8 p.m. on Saturday, Aug. 1. With only two performances left, this is one concert you don’t want to miss. The final concert of the season will take place on Friday, Aug. 7, with a perfor