Apply Style/Proc rubocop rule

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

.rubocop.yml

@@ -403,6 +403,9 @@ Style/OrAssignment:
 Style/PercentLiteralDelimiters:
   Enabled: true
 
+Style/Proc:
+  Enabled: true
+
 Style/RedundantFreeze:
   Enabled: true
 

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

app/models/poll/question.rb

@@ -26,7 +26,7 @@ class Poll::Question < ApplicationRecord
 
   validates_translation :title, presence: true, length: { minimum: 4 }
   validates :author, presence: true
-  validates :poll_id, presence: true, if: Proc.new { |question| question.poll.nil? }
+  validates :poll_id, presence: true, if: proc { |question| question.poll.nil? }
 
   validates_associated :votation_type
   accepts_nested_attributes_for :question_answers, reject_if: :all_blank, allow_destroy: true

config/initializers/devise.rb

@@ -12,7 +12,7 @@ Devise.setup do |config|
   # Configure the e-mail address which will be shown in Devise::Mailer,
   # note that it will be overwritten if you use your own mailer class
   # with default "from" parameter.
-  config.mailer_sender = Proc.new { "'#{Setting["mailer_from_name"]}' <#{Setting["mailer_from_address"]}>" }
+  config.mailer_sender = proc { "'#{Setting["mailer_from_name"]}' <#{Setting["mailer_from_address"]}>" }
 
   # Configure the class responsible to send e-mails.
   config.mailer = "DeviseMailer"