Update solidity version on test helpers

test/helpers/BasicTokenMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/token/BasicToken.sol';

test/helpers/BurnableTokenMock.sol

-pragma solidity ^0.4.13;
+pragma solidity ^0.4.18;
 
 import '../../contracts/token/BurnableToken.sol';
 

test/helpers/CappedCrowdsaleImpl.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/crowdsale/CappedCrowdsale.sol';

test/helpers/DayLimitMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 import "../../contracts/DayLimit.sol";
 
 contract DayLimitMock is DayLimit {

test/helpers/DetailedERC20Mock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 import '../../contracts/token/StandardToken.sol';
 import '../../contracts/token/DetailedERC20.sol';

test/helpers/ERC23TokenMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/token/BasicToken.sol';

test/helpers/FinalizableCrowdsaleImpl.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/crowdsale/FinalizableCrowdsale.sol';

test/helpers/ForceEther.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 // @title Force Ether into a contract.
 // @notice  even

test/helpers/HasNoEtherTest.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 import "../../contracts/ownership/HasNoEther.sol";
 

test/helpers/InsecureTargetBounty.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import {Bounty, Target} from "../../contracts/Bounty.sol";

test/helpers/LimitBalanceMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/LimitBalance.sol';

test/helpers/PausableMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/lifecycle/Pausable.sol';

test/helpers/PausableTokenMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 import '../../contracts/token/PausableToken.sol';
 

test/helpers/PullPaymentMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/payment/PullPayment.sol';

test/helpers/ReentrancyAttack.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 contract ReentrancyAttack {
 

test/helpers/ReentrancyMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 import '../../contracts/ReentrancyGuard.sol';
 import './ReentrancyAttack.sol';

test/helpers/RefundableCrowdsaleImpl.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/crowdsale/RefundableCrowdsale.sol';

test/helpers/SafeERC20Helper.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 import '../../contracts/token/ERC20.sol';
 import '../../contracts/token/SafeERC20.sol';

test/helpers/SafeMathMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/math/SafeMath.sol';

test/helpers/SecureTargetBounty.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import {Bounty, Target} from "../../contracts/Bounty.sol";

test/helpers/SplitPaymentMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 import '../../contracts/payment/SplitPayment.sol';
 

test/helpers/StandardTokenMock.sol

-pragma solidity ^0.4.11;
+pragma solidity ^0.4.18;
 
 
 import '../../contracts/token/StandardToken.sol';