Update old test for currently-accepted ancient TZ behavior.

The logic here ends up using a local mean time (LMT) calculation
to determine what the timezone offset would be in pre-timezone
eras. In this case, America/Chicago at year 0 gets an offset of
-05:50:36 rather than the -06:00 you would expect with simple
timezone calculation. Since we trust Joda, we go with the LMT
result here rather than forcing logic to match MRI's results.

spec/regression/GH-652_time_add_spec.rb

@@ -11,6 +11,6 @@
   it 'handles BC time correctly' do
     time = Time.new(0)
     other_time = Time.new(2012, 5, 23, 12, 0, 0)
-    (time - other_time).should eq(-63504988500.0)
+    (time - other_time).should eq(-63504990564.0)
   end
 end