Don't depend on rubygems loading thread (for Mutex)

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -1,3 +1,4 @@
+require 'thread'
 require 'monitor'
 require 'set'
 require 'active_support/core_ext/module/synchronization'

activerecord/test/cases/locking_test.rb

@@ -1,3 +1,4 @@
+require 'thread'
 require "cases/helper"
 require 'models/person'
 require 'models/reader'

activesupport/lib/active_support/core_ext/module/synchronization.rb

@@ -1,3 +1,4 @@
+require 'thread'
 require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/array/extract_options'
 

activesupport/test/core_ext/module/synchronization_test.rb

@@ -1,3 +1,4 @@
+require 'thread'
 require 'abstract_unit'
 
 require 'active_support/core_ext/class/attribute_accessors'