contracts/chequebook: update for latest Solidity (#15425)

This changes behaviour: before if the owner/amount didn't match, it
resulted in a successful execution without doing anything; now it
results in a failed execution using the revert opcode (remainder gas
is not consumed).

contracts/chequebook/contract/chequebook.sol

-import "mortal";
+pragma solidity ^0.4.18;
+
+import "https://github.com/ethereum/solidity/std/mortal.sol";
 
 /// @title Chequebook for Ethereum micropayments
-/// @author Daniel A. Nagy <daniel@ethdev.com>
+/// @author Daniel A. Nagy <daniel@ethereum.org>
 contract chequebook is mortal {
     // Cumulative paid amount in wei to each beneficiary
     mapping (address => uint256) public sent;
@@ -21,26 +23,23 @@ contract chequebook is mortal {
         uint8 sig_v, bytes32 sig_r, bytes32 sig_s) {
         // Check if the cheque is old.
         // Only cheques that are more recent than the last cashed one are considered.
-        if(amount <= sent[beneficiary]) return;
+        require(amount > sent[beneficiary]);
         // Check the digital signature of the cheque.
-        bytes32 hash = sha3(address(this), beneficiary, amount);
-        if(owner != ecrecover(hash, sig_v, sig_r, sig_s)) return;
+        bytes32 hash = keccak256(address(this), beneficiary, amount);
+        require(owner == ecrecover(hash, sig_v, sig_r, sig_s));
         // Attempt sending the difference between the cumulative amount on the cheque
         // and the cumulative amount on the last cashed cheque to beneficiary.
         uint256 diff = amount - sent[beneficiary];
         if (diff <= this.balance) {
 	    // update the cumulative amount before sending
             sent[beneficiary] = amount;
-            if (!beneficiary.send(diff)) {
-                // Upon failure to execute send, revert everything
-                throw;
-            }
+            beneficiary.transfer(diff);
         } else {
             // Upon failure, punish owner for writing a bounced cheque.
             // owner.sendToDebtorsPrison();
             Overdraft(owner);
             // Compensate beneficiary.
-            suicide(beneficiary);
+            selfdestruct(beneficiary);
         }
     }
 }