Fixed to work with bs V3

The current lessc.php throws errors with bs v3 mixins that don't contain default values.

The latest version fixes this.
Just wanted to merge the pull-request but got a warning on installing the plugin. The param->get() does not match == "-1". I wonder why do check for == "-1" here? The get function itself provides a default fallback e.g. $this->params->get('paramName', 'defaultValue').  So in this case $this->params->get('adminlessc', 'lessc-0.3.9')