Update fflib_SObjectSelector.cls

Changed the default value of getOrderBy() as not all objects have a 'Name' field (e.g. OpportunityContactRole, CampaignMember).
Thanks for this, though in its current for form, it will break backwards compatibility with the library. 

I see @capeterson has already mentioned this (though i cannot now see the comment), but i recommend we use isNameField method to dynamically discover the correct Name field (that way the response to this method will remain Name for the majority of existing user cases). 

As Chris also points out a good separation of concern would be to put a getNamedField method on the fflib_SObjectDescribe class and call this from the getOrderBy method. We would want to also consider performance here so some caching of the result will likely be important.