Update metadata[:map]

AutoPrefixer has support for SourceMaps but in Sprockets 4.x there is a `metadata[:map]` onto which we tap with Jekyll-Assets (for efficiency), we then transform that, update the file names and spit it out as a `.map` file, this works quite well, because Sprockets itself even updates it as it Uglifies, or uses CSS.

It would be nice if AutoPrefixer also did this, this way our maps don't potentially end up out of sync.