add previousOwner field to OwnershipTransferred event

contracts/ownership/Ownable.sol

@@ -10,7 +10,7 @@ contract Ownable {
   address public owner;
 
 
-  event OwnershipTransferred(address indexed newOwner);
+  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
 
 
   /**