Fix version error for truffle-flattener. (#794)

contracts/crowdsale/validation/IndividuallyCappedCrowdsale.sol

-pragma solidity ^ 0.4.18;
+pragma solidity ^0.4.18;
 
 import "../../math/SafeMath.sol";
 import "../Crowdsale.sol";

contracts/crowdsale/validation/WhitelistedCrowdsale.sol

-pragma solidity ^ 0.4.18;
+pragma solidity ^0.4.18;
 
 import "../Crowdsale.sol";
 import "../../ownership/Ownable.sol";