style(rubocop): fix all safe auto correctable offenses

script/generate_initial_config.rb

@@ -1,23 +1,22 @@
 #!/usr/bin/env ruby
-require File.expand_path('../../lib/postal/config', __FILE__)
-require 'openssl'
-require 'securerandom'
-require 'fileutils'
+require File.expand_path("../lib/postal/config", __dir__)
+require "openssl"
+require "securerandom"
+require "fileutils"
 
 unless File.directory?(Postal.config_root)
   FileUtils.mkdir_p(Postal.config_root)
 end
 
 unless File.exist?(Postal.config_file_path)
-  content = File.read(Postal.app_root.join('config', 'postal.example.yml'))
-  content.gsub!('{{secretkey}}', SecureRandom.hex(128))
-  File.open(Postal.config_file_path, 'w') { |f| f.write(content) }
+  content = File.read(Postal.app_root.join("config", "postal.example.yml"))
+  content.gsub!("{{secretkey}}", SecureRandom.hex(128))
+  File.write(Postal.config_file_path, content)
   puts "Created example config file at #{Postal.config_file_path}"
 end
 
-unless File.exists?(Postal.signing_key_path)
+unless File.exist?(Postal.signing_key_path)
   key = OpenSSL::PKey::RSA.new(1024).to_s
-  File.open(Postal.signing_key_path, 'w') { |f| f.write(key) }
+  File.write(Postal.signing_key_path, key)
   puts "Created new signing key for DKIM & HTTP requests"
 end
-