change approve() to conform to ERC20. Fix #438

contracts/token/StandardToken.sol

@@ -49,7 +49,7 @@ contract StandardToken is ERC20, BasicToken {
     //  allowance to zero by calling `approve(_spender, 0)` if it is not
     //  already 0 to mitigate the race condition described here:
     //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
-    require((_value == 0) || (allowed[msg.sender][_spender] == 0));
+    // require((_value == 0) || (allowed[msg.sender][_spender] == 0));
 
     allowed[msg.sender][_spender] = _value;
     Approval(msg.sender, _spender, _value);