Fix Sass load path

Throws "Fatal error: Access level to Assetic\Filter\Sass\SassFilter::$loadPaths must be protected (as in class Assetic\Filter\Sass\BaseSassFilter) or weaker"
Since it’s already available thorugh "BaseSassFilter", we don’t need it here.
Merge pull request #661 from niksy/fix-sass-loadpath

Fix Sass load path