Fix Migrations contract compile warnings

packages/concordia-contracts/contracts/Migrations.sol

@@ -1,10 +1,11 @@
+//SPDX-License-Identifier: MIT
 pragma solidity 0.7.1;
 
 contract Migrations {
     address public owner;
     uint public last_completed_migration;
 
-    constructor() public {
+    constructor() {
         owner = msg.sender;
     }