Update Less.php

Another fix for compatibility for PHP versions lower than 5.3.

Got error when saving settings from Shoestrap settings menu under PHP 5.2.x

Wrong parameters for Exception([string $exception [, long $code ] less.php 6205

change
```
parent::__construct($message, $code, $previous);
```
to
```
parent::__construct($message, $code);
```
Not sure if this changes anything else, but it gets rid of the error for me on php 5.2.17