Casey’s General Stores posted sharply higher earnings in the fourth quarter of fiscal 2015, but falling fuel prices cut into overall revenues.The Ankeny-based convenience store chain recorded net income of $41.3 million, or $1.05 per common share, in the final quarter of fiscal 2015, compared with $20.9 million, or 54 cents per share, in the same quarter of fiscal 2014.Revenue for the quarter fell to $1.7 billion from $1.9 billion in the same period a year earlier.