Fix server path configuration in development
This fixes path problems when setting up a local development environment. In dev, Fog uses the local storage provider and Indexer assumes all operations will result in files being written to `Pusher.server_path + "/gems"`. With Fog in the mix, this actually becomes `Pusher.server_path + s3_bucket + "/gems"`. (`Pusher.server_path` defaults to the `server` directory.) Removing the s3_bucket name from `rubygems.yml` and the `'gems'` from `Indexer#fog` means that gems and index updates are served out of the `Pusher.server_path` as expected. No tests for this as the behavior only occurs in the development environment.