Use address(this) instead of this (#1134)

* Use address(this) instead of this

It's preferred to use `address(this)` vs `this`

* Change to address(this) vs this

contracts/token/ERC20/TokenTimelock.sol

@@ -41,7 +41,7 @@ contract TokenTimelock {
     // solium-disable-next-line security/no-block-members
     require(block.timestamp >= releaseTime);
 
-    uint256 amount = token.balanceOf(this);
+    uint256 amount = token.balanceOf(address(this));
     require(amount > 0);
 
     token.safeTransfer(beneficiary, amount);

contracts/token/ERC20/TokenVesting.sol

@@ -87,7 +87,7 @@ contract TokenVesting is Ownable {
     require(revocable);
     require(!revoked[_token]);
 
-    uint256 balance = _token.balanceOf(this);
+    uint256 balance = _token.balanceOf(address(this));
 
     uint256 unreleased = releasableAmount(_token);
     uint256 refund = balance.sub(unreleased);
@@ -112,7 +112,7 @@ contract TokenVesting is Ownable {
    * @param _token ERC20 token which is being vested
    */
   function vestedAmount(ERC20Basic _token) public view returns (uint256) {
-    uint256 currentBalance = _token.balanceOf(this);
+    uint256 currentBalance = _token.balanceOf(address(this));
     uint256 totalBalance = currentBalance.add(released[_token]);
 
     if (block.timestamp < cliff) {