Revert change on ActiveRecord::Relation#order method that prepends new
order on the old ones

The previous behavior added a major backward incompatibility since it
impossible to have a upgrade path without major changes on the
application code.

We are taking the most conservative path to be consistent with the idea
of having a smoother upgrade on Rails 4.

We are reverting the behavior for what was in Rails 3.x and,
if needed, we will implement a new API to prepend the order clauses in
Rails 4.1.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/relation/query_methods.rb
	guides/source/upgrading_ruby_on_rails.md