Replace `render "public/404.html"` with `render file: "public/404.html"`

This change is backward-compatible, so it shouldn't break existing any
existing app.

Previously, calling render "foo/bar" in a controller action was
equivalent to render file: "foo/bar". In Rails 4.2, this has been
changed to mean render template: "foo/bar" instead.

See http://guides.rubyonrails.org/4_2_release_notes.html#render-with-a-string-argument