covnert TokenTimelock into initializers

contracts/mocks/TokenTimelockMock.sol

@@ -0,0 +1,14 @@
+pragma solidity ^0.4.24;
+
+import "../Initializable.sol";
+import "../token/ERC20/TokenTimelock.sol";
+
+contract TokenTimelockMock is Initializable, TokenTimelock {
+  constructor(
+    IERC20 token,
+    address beneficiary,
+    uint256 releaseTime
+  ) public {
+    TokenTimelock.initialize(token, beneficiary, releaseTime);
+  }
+}

contracts/token/ERC20/TokenTimelock.sol

@@ -1,5 +1,6 @@
 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./SafeERC20.sol";
 
 
@@ -8,7 +9,7 @@ import "./SafeERC20.sol";
  * @dev TokenTimelock is a token holder contract that will allow a
  * beneficiary to extract the tokens after a given release time
  */
-contract TokenTimelock {
+contract TokenTimelock is Initializable {
   using SafeERC20 for IERC20;
 
   // ERC20 basic token contract being held
@@ -20,12 +21,13 @@ contract TokenTimelock {
   // timestamp when token release is enabled
   uint256 private _releaseTime;
 
-  constructor(
+  function initialize(
     IERC20 token,
     address beneficiary,
     uint256 releaseTime
   )
     public
+    initializer
   {
     // solium-disable-next-line security/no-block-members
     require(releaseTime > block.timestamp);

test/token/ERC20/TokenTimelock.test.js

@@ -8,8 +8,8 @@ require('chai')
   .use(require('chai-bignumber')(BigNumber))
   .should();
 
-const ERC20Mintable = artifacts.require('ERC20Mintable');
-const TokenTimelock = artifacts.require('TokenTimelock');
+const ERC20Mintable = artifacts.require('ERC20MintableMock');
+const TokenTimelock = artifacts.require('TokenTimelockMock');
 
 contract('TokenTimelock', function ([_, minter, beneficiary]) {
   const amount = new BigNumber(100);