Coerce the ip address to a string

Leaving this as an object for JSON.generate to deal with was causing an IOError to be thrown when a request was serialized to send to airgun (similar to here - https://thin.lighthouseapp.com/projects/7212/tickets/114-requestenvto_json-fails) - when to_s is called on a `ActionDispatch::RemoteIp::GetIp`, the `env['action_dispatch.remote_ip']` all the env data (such as  `"rack.errors"=>#<IO:<STDERR>>`) is then not included in the hash sent to your servers.