Fix a transient failure in...

Fix a transient failure in db/post_migrate/20170717111152_cleanup_move_system_upload_folder_symlink.rb where symlink already exists
Signed-off-by: Rémy Coutable <remy@rymai.me>

db/post_migrate/20170717111152_cleanup_move_system_upload_folder_symlink.rb

@@ -20,7 +20,7 @@ class CleanupMoveSystemUploadFolderSymlink < ActiveRecord::Migration
   def down
     if File.directory?(new_directory)
       say "Symlinking #{old_directory} -> #{new_directory}"
-      FileUtils.ln_s(new_directory, old_directory)
+      FileUtils.ln_s(new_directory, old_directory) unless File.exist?(old_directory)
     else
       say "#{new_directory} doesn't exist, skipping."
     end