They may be older, more weathered and in at least one case, a lot grayer. But the “Jackass” gang — that merry band of Gen-X rascals — has hardly lost a step. “Jackass Forever,” the group’s fourth feature film and first since 2010’s “Jackass 3D,” is a riot of laughs amid a symphony of pain. The delivery system hasn’t changed a whole lot since “Jackass” first premiered on MTV in 2000: randy skate-type crew loves nothing more than subjecting themselves and each other to varying levels of unpleasant physical pranks, all in good fun and delivered with a laugh. But there’s still something oddly char…