Updated PausableCrowdsale to require solc ^0.4.24 (#1567)

(cherry picked from commit 73cbad2f)

contracts/crowdsale/validation/PausableCrowdsale.sol

-pragma solidity ^0.4.18;
+pragma solidity ^0.4.24;
 
 import "../Crowdsale.sol";
 import "../../lifecycle/Pausable.sol";

contracts/mocks/PausableCrowdsaleImpl.sol

-pragma solidity ^0.4.18;
+pragma solidity ^0.4.24;
 
 import "../token/ERC20/ERC20.sol";
 import "../crowdsale/validation/PausableCrowdsale.sol";