Update Mailchimp API to 2.4

[nabossha]

it would be nice having an update of the included Mailchimp-API to have the ability for passing the subscribers language to Mailchimp.

see: drewm/mailchimp-api@b04fa5f#diff-4658eb423d377d316ca4cdeb41f5c5da

after updating the API the subscribers language can be passed to Mailchimp by using this code within the MailchimpHook-class:

$params['data']['language'] = $GLOBALS['TSFE']->config['config']['language'];

[liayn]

This should be done together with #30 maybe?

[Tuurlijk]

@liayn No, please do a release first. Then a minor update for the new api usage. I tried, but it's not a drop in replacement.