Simplify default source map naming function

Right now, the current [default source map naming function](https://github.com/gruntjs/grunt-contrib-uglify/blob/master/tasks/uglify.js#L18-L21) splits the directory and filename just to join them again.

I guess it made sense before [PR184](https://github.com/gruntjs/grunt-contrib-uglify/pull/184/files) but not so much now.