Update greeter contract for modern solc

src/main/solidity/greeter/Greeter.sol

@@ -1,40 +1,38 @@
-pragma solidity ^0.4.25;
-
-// Modified Greeter contract. Based on example at https://www.ethereum.org/greeter.
+pragma solidity >=0.4.22 <0.6.0;
 
 contract Mortal {
-    /* Define variable owner of the type address*/
+    /* Define variable owner of the type address */
     address owner;
 
-    /* this function is executed at initialization and sets the owner of the contract */
-    constructor () public { owner = msg.sender; }
+    /* This constructor is executed at initialization and sets the owner of the contract */
+    constructor() public { owner = msg.sender; }
 
     /* Function to recover the funds on the contract */
-    function kill() public { if (msg.sender == owner) selfdestruct(owner); }
+    function kill() public { if (msg.sender == owner) selfdestruct(msg.sender); }
 }
 
 contract Greeter is Mortal {
-    /* define variable greeting of the type string */
+    /* Define variable greeting of the type string */
     string greeting;
 
-    /* this runs when the contract is executed */
-    constructor (string _greeting) public {
+    /* This runs when the contract is executed */
+    constructor(string memory _greeting) public {
         greeting = _greeting;
     }
 
-    function newGreeting(string _greeting) public {
+    function newGreeting(string memory _greeting) public {
         emit Modified(greeting, _greeting, greeting, _greeting);
         greeting = _greeting;
     }
 
-    /* main function */
-    function greet() public constant returns (string)  {
+    /* Main function */
+    function greet() public view returns (string memory) {
         return greeting;
     }
 
     /* we include indexed events to demonstrate the difference that can be
-    captured versus non-indexed */
+captured versus non-indexed */
     event Modified(
-            string indexed oldGreetingIdx, string indexed newGreetingIdx,
-            string oldGreeting, string newGreeting);
-}
+        string indexed oldGreetingIdx, string indexed newGreetingIdx,
+        string oldGreeting, string newGreeting);
+}