Attachment size validation - Fixed the number helper condition issue

In the original code, there is support added for Rails 4.0+ to show the size validation error message in KB/MB instead of Bytes. Unfortunately it was not working because of the wrong condition.

The condition checks `ActiveRecord::NumberHelper` is defined or not. But there is no such a class `ActiveRecord::NumberHelper`. 

I've changed the condition to `ActiveSupport::NumberHelper` using which the code is humanising size on line no. 75.

Now it is working.