Remove rails.png.

No reason to have a file, we've got data-URIs now! It didn't make sense
to me that you no longer have to remove public/index.html, but you still
need to remove rails.png. This means that you won't have to any more
when making a new Rails app.
adding .gigkeep to default app/assets/images
Merge pull request #10056 from steveklabnik/remove_rails_png

Remove rails.png.