Use address type

contracts/token/MintableToken.sol

@@ -35,7 +35,7 @@ contract MintableToken is StandardToken, Ownable {
     totalSupply = totalSupply.add(_amount);
     balances[_to] = balances[_to].add(_amount);
     Mint(_to, _amount);
-    Transfer(0x0, _to, _amount);
+    Transfer(address(0), _to, _amount);
     return true;
   }