convert TokenVesting to initializers

contracts/mocks/TokenVestingMock.sol

@@ -0,0 +1,22 @@
+pragma solidity ^0.4.24;
+
+import "../Initializable.sol";
+import "../drafts/TokenVesting.sol";
+
+contract TokenVestingMock is Initializable, TokenVesting {
+  constructor(
+    address beneficiary,
+    uint256 start,
+    uint256 cliffDuration,
+    uint256 duration,
+    bool revocable
+  ) public {
+    TokenVesting.initialize(
+      beneficiary,
+      start,
+      cliffDuration,
+      duration,
+      revocable
+    );
+  }
+}