Apply Style/Proc rubocop rule

While I tend to use `Proc.new`, using `proc` is shorter and more
consistent since we also use `lambda`.

app/mailers/application_mailer.rb

@@ -1,6 +1,6 @@
 class ApplicationMailer < ActionMailer::Base
   helper :settings
   helper :application
-  default from: Proc.new { "#{Setting["mailer_from_name"]} <#{Setting["mailer_from_address"]}>" }
+  default from: proc { "#{Setting["mailer_from_name"]} <#{Setting["mailer_from_address"]}>" }
   layout "mailer"
 end