Update Mailgun adapter for the new mail adapter API

Summary: Ref T920. Ref T5969. Update the Mailgun adapter for the API changes and add a timeout.

Test Plan: Configured Mailgun as a mailer, sent mail with subject/to/cc/headers/html/attachments using bin/mail send-test.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5969, T920

Differential Revision: https://secure.phabricator.com/D19959