fix thumb resize_to_limit

app/uploaders/gallery_uploader.rb

@@ -50,7 +50,7 @@ class GalleryUploader < CarrierWave::Uploader::Base
   # end
   
   version :thumb do
-    process :resize_to_limit => [200, 0]
+    process :resize_to_limit => [200, nil]
   end
 
   version :theater do