make transfer func compatible to eip20 standard

As discussed in ethereum/solidity#4116
Due to solidity compiler change after v0.4.22, this wrong transfer function will cause serious issues.
We (SECBIT.IO) found many people followed this tutorial on ethereum.org.
SHOULD fix this ASAP!

solidity/token-advanced.sol

@@ -82,8 +82,9 @@ contract TokenERC20 {
      * @param _to The address of the recipient
      * @param _value the amount to send
      */
-    function transfer(address _to, uint256 _value) public {
+    function transfer(address _to, uint256 _value) public returns (bool success) {
         _transfer(msg.sender, _to, _value);
+        return true;
     }
 
     /**