Merge pull request #4358 from castlerock/remove_plus_minus_support_for_ruby18

revert "fixing some tests on ruby 1.9.3" 

activesupport/lib/active_support/core_ext/date_time/calculations.rb

@@ -79,29 +79,6 @@ def end_of_day
     change(:hour => 23, :min => 59, :sec => 59)
   end
 
-  # 1.9.3 defines + and - on DateTime, < 1.9.3 do not.
-  if DateTime.public_instance_methods(false).include?(:+)
-    def plus_with_duration(other) #:nodoc:
-      if ActiveSupport::Duration === other
-        other.since(self)
-      else
-        plus_without_duration(other)
-      end
-    end
-    alias_method :plus_without_duration, :+
-    alias_method :+, :plus_with_duration
-
-    def minus_with_duration(other) #:nodoc:
-      if ActiveSupport::Duration === other
-        plus_with_duration(-other)
-      else
-        minus_without_duration(other)
-      end
-    end
-    alias_method :minus_without_duration, :-
-    alias_method :-, :minus_with_duration
-  end
-
   # Adjusts DateTime to UTC by adding its offset value; offset is set to 0
   #
   # Example: