Fix specs for Time#-

This has been proposed upstream as ruby/spec#1262

Author: herwinw

spec/core/time/minus_spec.rb

@@ -113,7 +113,7 @@
 
   it "does not return a subclass instance" do
     c = Class.new(Time)
-    x = c.now + 1
+    x = c.now - 1
     x.should be_an_instance_of(Time)
   end