Using Gibbon on Heroku for MailChimp

Question

I am using Gibbon to automatically send a new user's email to Mailchimp. In my Heroku logs I get this error message:

  Gibbon::MailChimpError (Signup disabled for "a@a.com"):

My gibbon.rb has my Mailchimp API key:

Gibbon::API.api_key = "My API key is in here ..."
Gibbon::API.timeout = 15
Gibbon::API.throws_exceptions = true

My controller calls this action:

  def subscribe

      @list_id = "123456789"    # My Mailchimp list ID
      gb = Gibbon::API.new

      gb.lists.subscribe({
        :id => @list_id,
    #    :email => {:email => params[:email][:address]}
        :email => {:email => "a@a.com"}        
        })
      redirect_to home_membership_path
  end

Also, in my development environment I get this error message which may or may not be a clue:

OpenSSL::SSL::SSLError in HomeController#subscribe

SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed

All help appreciated!