transform Ownable to initializers

contracts/mocks/OwnableMock.sol

+pragma solidity ^0.4.24;
+
+import { Ownable } from '../ownership/Ownable.sol';
+
+contract OwnableMock is Ownable {
+
+  constructor() {
+    initialize();
+  }
+
+}

contracts/ownership/Ownable.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 
 /**
  * @title Ownable
  * @dev The Ownable contract has an owner address, and provides basic authorization control
  * functions, this simplifies the implementation of "user permissions".
  */
-contract Ownable {
+contract Ownable is Initializable {
   address private _owner;
 
 
@@ -21,7 +22,7 @@ contract Ownable {
    * @dev The Ownable constructor sets the original `owner` of the contract to the sender
    * account.
    */
-  constructor() public {
+  function initialize() public initializer {
     _owner = msg.sender;
   }
 

test/ownership/Ownable.test.js

+const { EVMRevert } = require('../helpers/EVMRevert');
+const { expectThrow } = require('../helpers/expectThrow');
+
 const { shouldBehaveLikeOwnable } = require('./Ownable.behavior');
 
-const Ownable = artifacts.require('Ownable');
+const Ownable = artifacts.require('OwnableMock');
 
 contract('Ownable', function ([_, owner, ...otherAccounts]) {
   beforeEach(async function () {
     this.ownable = await Ownable.new({ from: owner });
   });
 
+  it('cannot be reinitialized', async function () {
+    await expectThrow(this.ownable.initialize(), EVMRevert);
+  });
+
   shouldBehaveLikeOwnable(owner, otherAccounts);
 });