Ruby 2.4 compatibility (don't use Fixnum anymore) (#2401)

* Ruby 2.4.0 compatibility (doen't use Fixnum anymore)

* Fix typo

* Use Integer instead of 0.class

NEWS

@@ -36,6 +36,7 @@
 * Improvement: S3 storage option `:s3_prefixes_in_alias`. (#2287)
 * Improvement: Fog option `:fog_public` can be a lambda. (#2302)
 * Improvement: One fewer warning on JRuby. (#2352)
+* Ruby 2.4.0 compatibility (doesn't use Fixnum anymore)
 
 5.1.0 (2016-08-19):
 

lib/paperclip/storage/fog.rb

@@ -184,7 +184,7 @@ module Paperclip
       private
 
       def convert_time(time)
-        if time.is_a?(Fixnum)
+        if time.is_a?(Integer)
           time = Time.now + time
         end
         time