readme: update badges

Updates the badges to the flat versions. Also adds a `downloads` badge. The bountysource badge can be updated after https://github.com/badges/shields/pull/307 is merged in. Hopefully this is helpful. Thanks!

edit: here's a preview -- https://github.com/eslint/eslint/blob/bd8128122b2e68485c59631f6d2d0166b1511bd0/README.md