405 Method Not Allowed on EU domains: drop /v3 from the endpoint

## The problem
Issue mailgun/mailgun-js#423 (closed, 16 comments): Sending through an EU-region domain fails with `[Error: Method Not Allowed]`, status 405, on `mg.messages.create(...)`. The client was created with the default endpoint and a `pubkey-` public key, and the domain lives in the EU region.

## What fixed it
Sending through an EU-region Mailgun domain failing with 405 Method Not Allowed: the default endpoint 405s on EU domains. Fix: create the client with the EU endpoint explicitly and without the v3 path segment: url: "https://api.eu.mailgun.net" in the mailgun.client() options. The default endpoint is the US one; on EU domains it returns 405. Root cause confirmed by the maintainer.