Resolve faraday deprecation warning
Resolve faraday deprecation warning

```
WARNING: `Faraday::Connection#basic_auth` is deprecated; it will be
removed in version 2.0.  While initializing your connection, use
`#request(:basic_auth, ...)` instead.  See
https://lostisland.github.io/faraday/middleware/authentication for more
usage info.
```

seven1m/mailchimp3