using defined Puma config

[JackDanger]

This changes threads the gemstash_env.config options for Puma through to the Puma::CLI.new call.

Prior to this change :puma_workers and :puma_threads were ignored when booting Gemstash via the CLI

[phanle]

@olleolleolle Let me know if further updates are required.

[bronzdoc]

Would like to test this before merge 😬