Add support for Puma 2.8.0

Puma 2.8.0 renamed the ``worker_boot`` option to
``before_worker_boot``[1]. This commit detects which option name is in
use and adds a hook to the correct one.

1: https://github.com/puma/puma/commit/e5150f07927e96aa5aae8a65db116472de1d9d63