Switch from unicorn to puma

Procfile

@@ -1 +1 @@
-web: bundle exec unicorn -p $PORT -c ./config/unicorn.rb
+web: bundle exec puma -C config/puma.rb

config/puma.rb

@@ -1,19 +1,24 @@
+# frozen_string_literal: true
+
+# Overview of config:
+# https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server
+
 # Puma can serve each request in a thread from an internal thread pool.
 # The `threads` method setting takes two numbers: a minimum and maximum.
 # Any libraries that use thread pools should be configured to match
 # the maximum value specified for Puma. Default is set to 5 threads for minimum
 # and maximum; this matches the default thread size of Active Record.
 #
-threads_count = ENV.fetch("RAILS_MAX_THREADS") { 5 }
+threads_count = ENV.fetch('RAILS_MAX_THREADS') { 5 }
 threads threads_count, threads_count
 
 # Specifies the `port` that Puma will listen on to receive requests; default is 3000.
 #
-port        ENV.fetch("PORT") { 3000 }
+port        ENV.fetch('PORT') { 3000 }
 
 # Specifies the `environment` that Puma will run in.
 #
-environment ENV.fetch("RAILS_ENV") { "development" }
+environment ENV.fetch('RAILS_ENV') { 'development' }
 
 # Specifies the number of `workers` to boot in clustered mode.
 # Workers are forked webserver processes. If using threads and workers together
@@ -21,14 +26,20 @@ environment ENV.fetch("RAILS_ENV") { "development" }
 # Workers do not work on JRuby or Windows (both of which do not support
 # processes).
 #
-# workers ENV.fetch("WEB_CONCURRENCY") { 2 }
+workers ENV.fetch('WEB_CONCURRENCY') { 2 }
 
 # Use the `preload_app!` method when specifying a `workers` number.
 # This directive tells Puma to first boot the application and load code
 # before forking the application. This takes advantage of Copy On Write
 # process behavior so workers use less memory.
 #
-# preload_app!
+preload_app!
+
+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
 
 # Allow puma to be restarted by `rails restart` command.
 plugin :tmp_restart