Deprecate Rails::Rack::LogTailer

It was being used by Rails for show the development logs but since Rails
4 it is not being used anymore on the framework.

This class seems to be private but it were part of the public API we are
deprecating before removing.

Author: rafaelfranca

railties/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Deprecate `Rails::Rack::LogTailer` with not replacement.
+
+    *Rafael Mendonça França*
+
 *   Add a generic --skip-gems options to generator
 
     This option is useful if users want to remove some gems like jbuilder,

railties/lib/rails/rack/log_tailer.rb

@@ -1,7 +1,11 @@
+require 'active_support/deprecation'
+
 module Rails
   module Rack
     class LogTailer
       def initialize(app, log = nil)
+        ActiveSupport::Deprecation.warn "LogTailer is deprecated and will be removed on Rails 5"
+
         @app = app
 
         path = Pathname.new(log || "#{::File.expand_path(Rails.root)}/log/#{Rails.env}.log").cleanpath