Fix bug with remove_previously_stored_#{column} callback, when updating in a transaction, and that process is rollback.
The old file will be delete, but the new file name can not store success into database.

## For example:

User.transaction do
  @user.avatar = new_file
  @user.save
  # new file stored
  # old file deleted with callback: remove_previously_stored_avatar
  # ... do bla bla
  raise ActiveRecord::Rollback
end
Merge pull request #1447 from huacnlee/fix_remove_previously_stored_in_transaction

Fix bug with remove_previously_stored_#{column} callback, when updating in a transaction, and that process is rollback