Puma: Enable support for TLS

Currently Workhorse talks to Puma via HTTP or an unencrypted UNIX socket:

In config/puma.rb, we're going to want this:

Puma natively supports SSL via OpenSSL: https://github.com/puma/puma#ssl-connection-support

We need to:

1. Configure SSL server key and certificate.
2. Add support for bind 'ssl://hostname'.