Add version string

[aral]

It would help to know that we’re running the latest version of Pebble (and to compare versions of Pebble on different machines), if it had a version string.

e.g.,

pebble --version

Or, barring that, it would be fine if it output its version string in the log output. e.g.,

Pebble 2021/03/23 12:19:15 Starting Pebble ACME server (version 2.3.1)