Added 60s timeout to Mailgun API calls

refs https://github.com/TryGhost/Toolbox/issues/164

- see referenced issue for more context but Ghost sometimes has issues
  with the email analytics job getting stuck
- we don't provide a timeout to the Mailgun library, so we just
  sit there idling for eternity if something between us and Mailgun is
  causing issues
- this commit adds a 60s timeout so we can at least error out and try
  again next time

ghost/mailgun-client/lib/mailgun-client.js

@@ -194,7 +194,8 @@ module.exports = class MailgunClient {
         return mailgun.client({
             username: 'api',
             key: mailgunConfig.apiKey,
-            url: baseUrl.origin
+            url: baseUrl.origin,
+            timeout: 60000
         });
     }