undo previous reverts to allow use of freebies-invitations template

- Another PR: https://github.com/GigaOM/go-recurly/pull/28 reverted our use of the new freebies template on Mandrill. So, until this fix, we'd be sending alerts templates for freebies invitees. 
- Not clear why it was reverted?
- This PR routes the emails to the desired template (`freebies-invitations`) and thus also adds the date calculation back as required by the new template.