Bumped required solidity version to 0.4.24. (#1295)

contracts/drafts/ERC1046/TokenMetadata.sol

-pragma solidity ^0.4.21;
+pragma solidity ^0.4.24;
 
 import "../../token/ERC20/IERC20.sol";
 

contracts/mocks/ERC20WithMetadataMock.sol

-pragma solidity ^0.4.21;
+pragma solidity ^0.4.24;
 
 import "../token/ERC20/ERC20.sol";
 import "../drafts/ERC1046/TokenMetadata.sol";

contracts/payment/ConditionalEscrow.sol

-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 import "./Escrow.sol";
 

contracts/payment/Escrow.sol

-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 import "../math/SafeMath.sol";
 import "../ownership/Ownable.sol";

contracts/payment/RefundEscrow.sol

-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 import "./ConditionalEscrow.sol";
 import "../ownership/Ownable.sol";