My second solution

application.rb

require 'guard'
require 'guard/guard'
require 'libnotify'
require 'thread'

module Guard

  class Development < Guard

    Sempahore = Mutex.new

    def self.last_update
      Sempahore.synchronize do
        @last_update ||= Time.now
      end
    end

    def self.update
      Sempahore.synchronize do
        @last_update = Time.now
      end
    end

    # Called on file(s) modifications
    def run_on_change(paths)
      self.class.update
    end

    def self.try

      Thread::new do # http://www.ruby-doc.org/core/classes/Thread.html
        ::Guard.setup
        ::Guard.start(:guardfile_contents => "
            guard 'development' do
              watch(%r{^app/(.+)\.rb})
            end
          ")
      end
    end
  end
end

Guard::Development.try if Rails.env.development? && defined?(Rails::Server)

bootstrap.rb

      initializer :set_clear_dependencies_hook do
        unless config.cache_classes
          changed_at = Proc.new{ Guard::Development.last_update }
          last_change = changed_at.call

          ActionDispatch::Callbacks.before do
            change = changed_at.call                        
            if change > last_change
              last_change = change
              ActiveSupport::Notifications.instrument("server.reloading") do
                ActiveSupport::DescendantsTracker.clear
                ActiveSupport::Dependencies.clear
              end
            end
          end
        end
      end