add missing prerenewal_notification type

[joprice]

The Notification Type seems to be missing the prerenewal_notification type: https://developers.recurly.com/reference/webhooks/#subscription-prerenewal. This results in Notification.detect failing to parse to a valid Type.

[joannasese]

Hi @joprice. Apologies for the delay here - missed your question completely. Thanks for alerting us. We'll get a PR up shortly.