This fixes a deprecation warning being thrown in Rails 4

In Rails 4 you should not do the following any more:

    has_many :children, order: configuration[:order], # Etc.

Instead it the order should be passed in a scope:

    has_many :children, -> { order configuration[:order] }, # Etc.

This pull request fixes that.

Note: I'm relying on `Rails.version` to make sure the gem keeps working in Rails 3. Not sure if the way I've implemented that is desirable.