Sometimes, what happens in Williamstown really oughta stay in Williamstown. That Berkshires burg with its annual summer-theater festival has been sending shows to Broadway left (The Elephant Man, a hit earlier this season) and right (The Visit, opening later this week). Last year’s The Bridges of Madison County started off at Williamstown, a siren call to stars looking for a summer fling or producers angling for a cheap tryout in country surroundings.