Update to mailchimp v2.0 api and handle api errors

When trying to authenticate as a restricted user (i.e. not owner or admin) the strategy blows up building the info hash since raw_info fails to get any data and returns nil.  This is due to mailchimp's restrictions on the helper/account-details api endpoint (http://apidocs.mailchimp.com/api/2.0/helper/account-details.php) which throws an error when accessed by restricted users.  This patch treats that as a failure to authenticate and calls fail!, allowing the consumer to handle it gracefully rather than throw a 500 or other error page.  I also updated the api call to use the new v2.0 api call since v1.3 is now deprecated.