ERC1271 is out of date

[cwhinfrey]

It looks like 2ccb26e reverted #2. Was that intentional? ERC1271 now specifies the following function signatur. It may be worth re-adding the switch to ERC1654 or updating the interface.

function isValidSignature(
    bytes memory _data, 
    bytes memory _signature)
    public
    view 
    returns (bytes4 magicValue);