Stop testing on 1.8.7, 1.9.2, rbx-18mode, and rbx-19mode.
Adding Rails 4 and its dependent gems.

Unfortunately we can't just use only activerecord, since it's not released as stand alone gem.

Also:
* Replacing all .scoped calls with .all
* Using new order syntax on AR associations
* Replaced deprecated finder methods
* Using to_a for comparing activerecord relations in specs.
* Using correct activerecord syntax in nested_set_scope method
Use only activerecord, not all of Rails.
attr_accessible was removed from activerecord.
Let specs run and fix deprecation warnings about default_scope.