Change `uint` to `uint256` (#1160)

This is a minor change, aligning to the same convention used in the other contracts.

contracts/mocks/BurnableTokenMock.sol

@@ -5,7 +5,7 @@ import "../token/ERC20/BurnableToken.sol";
 
 contract BurnableTokenMock is BurnableToken {
 
-  constructor(address _initialAccount, uint _initialBalance) public {
+  constructor(address _initialAccount, uint256 _initialBalance) public {
     balances[_initialAccount] = _initialBalance;
     totalSupply_ = _initialBalance;
   }