Add .DS_Store to .gitignore
Fix port defaulting

Ensure that the port defaulting behaves how we want.

Ports the following commits from our old customized version:
https://github.com/looker/puma/commit/fbeef9679244e06879ecf59b57c2d0c4923c7424
https://github.com/looker/puma/commit/e6647c30322610b78501e26f68f6793ecae418a3