Upgrade to latest version of Puma and revisit configuration

[samandmoore]

Currently, we don't have a Procfile for deployments to heroku, so heroku will use the default configuration for running the web server.

That's probably good though because the current version of our config/puma.rb probably isn't really heroku compatible. We should consider dropping in a different version of that file or removing it altogether if it's not really needed.

[jmileham]

config/puma.rb is now compatible with Heroku.