style(rubocop): fix all safe auto correctable offenses

config/boot.rb

@@ -1,11 +1,11 @@
-ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../Gemfile', __dir__)
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
 
 $stdout.sync = true
 $stderr.sync = true
 
-require 'bundler/setup' # Set up gems listed in the Gemfile.
+require "bundler/setup" # Set up gems listed in the Gemfile.
 
-require_relative '../lib/postal/config'
+require_relative "../lib/postal/config"
 Postal.check_config!
 
-ENV['RAILS_ENV'] = Postal.config.rails&.environment || 'development'
+ENV["RAILS_ENV"] = Postal.config.rails&.environment || "development"