Update to latest version

Signed-off-by: Richie Bendall <richiebendall@gmail.com>

s/Solidity.sol

@@ -1,7 +1,7 @@
-pragma solidity ^0.4.24;
+pragma solidity ^0.6.4;
 
 contract HelloWorld {
-    function render () public pure returns (string) {
+    function render () public pure returns (string memory) {
         return 'Hello World';
     }
 }