Remove CouchDB vhost configuration.

Use the full `_design/app/_rewrite` URL for the registry, as discussed in #1.

Before this change,
 - `http://localhost:15984/` was the CouchDB REST API,
 -  `http://127.0.0.1:15984/` was the npm registry REST API.

This commit removes the difference between localhost and 127.0.0.1.

The npm registry REST API must be accessed via
  - `http://{host:port}/_design/app/_rewrite`

/to: @rmg