Removes support to MRI 1.8.

Author: lucasallan

lib/ref.rb

@@ -19,12 +19,6 @@ module Ref
     if defined?(RUBY_ENGINE) && RUBY_ENGINE == 'rbx'
       # If using Rubinius set the implementation to use WeakRef since it is very efficient and using finalizers is not.
       require 'ref/weak_reference/weak_ref'
-    elsif defined?(::ObjectSpace::WeakMap)
-      # Ruby 2.0 has a working implementation of weakref.rb backed by the new ObjectSpace::WeakMap
-      require 'ref/weak_reference/weak_ref'
-    elsif defined?(::ObjectSpace._id2ref)
-      # If ObjectSpace can lookup objects from their object_id, then use the pure ruby implementation.
-      require 'ref/weak_reference/pure_ruby'
     else
       # Otherwise, wrap the standard library WeakRef class
       require 'ref/weak_reference/weak_ref'