Make contracts 4.24 compatible (#951)

* Make contracts 4.24 compatible

contracts/examples/RBACWithAdmin.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 import "../ownership/rbac/RBAC.sol";
 

contracts/examples/SampleCrowdsale.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 import "../crowdsale/validation/CappedCrowdsale.sol";
 import "../crowdsale/distribution/RefundableCrowdsale.sol";

contracts/examples/SimpleSavingsWallet.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 import "../ownership/Heritable.sol";
 

contracts/examples/SimpleToken.sol

@@ -1,4 +1,4 @@
-pragma solidity ^0.4.23;
+pragma solidity ^0.4.24;
 
 
 import "../token/ERC20/StandardToken.sol";