Remove time stubs after each test

Reverts 7abb6e00.

activerecord/test/cases/mixin_test.rb

@@ -10,10 +10,6 @@ class TouchTest < ActiveRecord::TestCase
     travel_to Time.now
   end
 
-  teardown do
-    travel_back
-  end
-
   def test_update
     stamped = Mixin.new
 

activesupport/lib/active_support/testing/time_helpers.rb

@@ -51,8 +51,14 @@ def unstub_object(stub)
 
     # Contains helpers that help you test passage of time.
     module TimeHelpers
+      def after_teardown
+        travel_back
+        super
+      end
+
       # Changes current time to the time in the future or in the past by a given time difference by
-      # stubbing +Time.now+, +Date.today+, and +DateTime.now+.
+      # stubbing +Time.now+, +Date.today+, and +DateTime.now+. The stubs are automatically removed
+      # at the end of the test.
       #
       #   Time.current     # => Sat, 09 Nov 2013 15:34:49 EST -05:00
       #   travel 1.day
@@ -74,6 +80,7 @@ def travel(duration, &block)
 
       # Changes current time to the given time by stubbing +Time.now+,
       # +Date.today+, and +DateTime.now+ to return the time or date passed into this method.
+      # The stubs are automatically removed at the end of the test.
       #
       #   Time.current     # => Sat, 09 Nov 2013 15:34:49 EST -05:00
       #   travel_to Time.zone.local(2004, 11, 24, 01, 04, 44)

activesupport/test/time_zone_test.rb

@@ -103,7 +103,6 @@ def test_today
     assert_equal Date.new(2000, 1, 1), ActiveSupport::TimeZone["Eastern Time (US & Canada)"].today
     travel_to(Time.utc(2000, 1, 2, 5)) # midnight Jan 2 EST
     assert_equal Date.new(2000, 1, 2), ActiveSupport::TimeZone["Eastern Time (US & Canada)"].today
-    travel_back
   end
 
   def test_tomorrow
@@ -115,7 +114,6 @@ def test_tomorrow
     assert_equal Date.new(2000, 1, 2), ActiveSupport::TimeZone["Eastern Time (US & Canada)"].tomorrow
     travel_to(Time.utc(2000, 1, 2, 5)) # midnight Jan 2 EST
     assert_equal Date.new(2000, 1, 3), ActiveSupport::TimeZone["Eastern Time (US & Canada)"].tomorrow
-    travel_back
   end
 
   def test_yesterday
@@ -127,7 +125,6 @@ def test_yesterday
     assert_equal Date.new(1999, 12, 31), ActiveSupport::TimeZone["Eastern Time (US & Canada)"].yesterday
     travel_to(Time.utc(2000, 1, 2, 5)) # midnight Jan 2 EST
     assert_equal Date.new(2000, 1, 1), ActiveSupport::TimeZone["Eastern Time (US & Canada)"].yesterday
-    travel_back
   end
 
   def test_travel_to_a_date