Use V2 of MailChimp API

Hey @braidn - 

If you have some time in the next week or so, do you want to take a run at this? We're currently using an old version of the mailchimp gem that communicates with an older (deprecated) version of the API. 

I'd like to upgrade to use https://rubygems.org/gems/mailchimp-api. 

cc: @joshnuss @DynamoMTL/mickeyren