node path api changed

[hthief]

related to nodejs/node#5348

when running cssmin task on nodev6 it returns
Warning: Path must be a string. Received undefined Use --force to continue.
In cases where previously didn't, in this case when the file isn't present, where as previously simply would create an empty file with the path given. Not sure if needed a fix or an update adhere to the stricter implementation of path.