Modify paperclip version from 4.2.0 to 3.5.4

It makes a warning message.

rake #=>
DEPRECATION WARNING: String based terminators are deprecated, please use
a lambda. (called from <class:Item> at
/some/where/app/app/models/item.rb:2)

ref.
  https://github.com/thoughtbot/paperclip/issues/1403
  https://github.com/thoughtbot/paperclip/pull/1404
  http://dev.mensfeld.pl/2014/05/upgrading-to-rails-4-1-from-rails-4-0-ruby-on-rails/