Add a basic Puma configuration file

Heroku recommends generating a config file rather than setting values inline in the Procfile:

https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server

Author: O-I

Diff for: Procfile

@@ -1 +1 @@
-web: bundle exec puma -t 5:5 -p ${PORT:-3000} -e ${RACK_ENV:-development}
+web: bundle exec puma -C config/puma.rb

Diff for: config/puma.rb

@@ -0,0 +1,15 @@
+workers Integer(ENV['WEB_CONCURRENCY'] || 2)
+threads_count = Integer(ENV['MAX_THREADS'] || 5)
+threads threads_count, threads_count
+
+preload_app!
+
+rackup      DefaultRackup
+port        ENV['PORT']     || 3000
+environment ENV['RACK_ENV'] || 'development'
+
+on_worker_boot do
+  # Worker specific setup for Rails 4.1+
+  # See: https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#on-worker-boot
+  ActiveRecord::Base.establish_connection
+end