Do not require railtie unless Rails is defined

Fixes #843

lib/paperclip.rb

@@ -46,11 +46,12 @@ require 'paperclip/validators'
 require 'paperclip/instance_methods'
 require 'paperclip/logger'
 require 'paperclip/helpers'
-require 'paperclip/railtie'
 require 'mime/types'
 require 'logger'
 require 'cocaine'
 
+require 'paperclip/railtie' if defined?(Rails)
+
 # The base module that gets included in ActiveRecord::Base. See the
 # documentation for Paperclip::ClassMethods for more useful information.
 module Paperclip