Merge pull request rspec#2010 from rspec/switch_reentrant_mutex_to_support

Switch to using re entrant mutex from rspec-support

Author: myronmarston

lib/rspec/core/memoized_helpers.rb

@@ -1,4 +1,4 @@
-require 'rspec/core/reentrant_mutex'
+require 'rspec/support/reentrant_mutex'
 
 module RSpec
   module Core
@@ -148,7 +148,7 @@ def __init_memoized
       class ThreadsafeMemoized
         def initialize
           @memoized = {}
-          @mutex = ReentrantMutex.new
+          @mutex = Support::ReentrantMutex.new
         end
 
         def fetch_or_store(key)