* Fix for default smtp certificate name
This fixes the defaut smtp certificate file name from smtp.crt to smtp.cert as it is used throughout the postal code.
This allows us to avoid messing around with changes to iptables and generally makes installation easier for everyone.
If you have an instance of postal before this, you will need to do a couple of things to ensure your installation continues to work.
1. You will need to specify a `fast_server.bind_address` in your postal.yml configuration file. This is the IP that you dedicated to the fast server.
2. You will need to allow Ruby to listen on privileged ports (sudo setcap 'cap_net_bind_service=+ep' /usr/bin/ruby2.3).
3. You will need to remove any iptables rules that you may have added previously to accomodate the older way of doing this.
