Default config.assets.digests to true in development

With https://github.com/rails/sprockets-rails/pull/149, assets will only be served if the request has a digest if `config.assets.digest = true` and `config.assets.raise_runtime_errors = true`.

This PR sets `config.assets.digest` to `true` by default and will help users avoid hardcoding paths that work in development but break in production.