apply realpath() when using pipeline dir

This applies [realpath()](http://php.net/manual/en/function.realpath.php) to the pipeline directory.

If you use "../min" as *pipeline_dir* the following path is returned
> assets/css/../min/61db2de2a54a3e83229e18b0612787ce.css

With this modification the result will be
> assets/min/61db2de2a54a3e83229e18b0612787ce.css