Rename serve_static_assets to serve_static_files.

I think the branches by Rails version should be less. `serve_static_files` or `serve_static_assets` are [default `true`](https://github.com/rails/rails/commit/2d04bdd86fb4a9c69e1ca1ffe92188a9ca4f88c8). So it might be better to remove this configuration section.