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.
Delegate connection to class

Rails 4 has deprecated the instance connection method, this adds a delegation method back to skip the deprecation warning
Merge pull request #184 from lexrupy/rails4

Delegate connection to class
Merge branch 'master' into rails4

Conflicts:
	.travis.yml
	lib/awesome_nested_set/awesome_nested_set.rb
	spec/awesome_nested_set_spec.rb
Merge pull request #193 from shevaun/master

Refactor challenge: improve code climate score
Refactored all_roots_valid?
Extracted validator predicates to Validators module.
Moved scope_string out into its own method.
Made the responsibilities of acts_as_nested_set clear.
Remove untested, unused sorted_each_with_level.
Removed untested method sorted_nested_set_options
Rename validators to validatable and introduce rebuildable.
Added pry debugging to the Gemfile.
Simplify logic in Tree#set_left_and_rights
Always order by the quoted_order_column_name
Made use of arel when constructing queries.