convert ERC20Migrator to initializers

contracts/drafts/ERC20Migrator.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "../token/ERC20/IERC20.sol";
 import "../token/ERC20/ERC20Mintable.sol";
 import "../token/ERC20/SafeERC20.sol";
@@ -32,7 +33,7 @@ import "../math/Math.sol";
  * await migrator.beginMigration(newToken.address);
  * ```
  */
-contract ERC20Migrator {
+contract ERC20Migrator is Initializable {
   using SafeERC20 for IERC20;
 
   /// Address of the old token contract
@@ -44,7 +45,7 @@ contract ERC20Migrator {
   /**
    * @param legacyToken address of the old token contract
    */
-  constructor(IERC20 legacyToken) public {
+  function initialize(IERC20 legacyToken) public initializer {
     require(legacyToken != address(0));
     _legacyToken = legacyToken;
   }

contracts/mocks/ERC20MigratorMock.sol

+pragma solidity ^0.4.24;
+
+import "../Initializable.sol";
+import "../drafts/ERC20Migrator.sol";
+
+
+contract ERC20MigratorMock is Initializable, ERC20Migrator {
+
+  constructor(IERC20 legacyToken) public {
+    ERC20Migrator.initialize(legacyToken);
+  }
+
+}

test/drafts/ERC20Migrator.test.js

 const { assertRevert } = require('../helpers/assertRevert');
 
 const ERC20Mock = artifacts.require('ERC20Mock');
-const ERC20Mintable = artifacts.require('ERC20Mintable');
-const ERC20Migrator = artifacts.require('ERC20Migrator');
+const ERC20Mintable = artifacts.require('ERC20MintableMock');
+const ERC20Migrator = artifacts.require('ERC20MigratorMock');
 
 const BigNumber = web3.eth.BigNumber;