If not use URL Rewrite, only run supported files through Munee when fixing relative paths in CSS Files

[meenie]

If not using URL Rewrite (.htaccess file), then Munee shouldn't run unsupported files through itself. Ref: https://github.com/meenie/munee/blob/master/src/Munee/Asset/Type/Css.php#L248

This is related to #79