Use Ruby's #include? to avoid relying on AS extension

The build has failed when running the date/time ext tests in isolation
due to the missing extension, so better than adding a require is using
just Ruby in this case.

https://travis-ci.org/rails/rails/jobs/46107954#L1077

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

@@ -48,7 +48,7 @@ def future?
 
     # Returns true if the date/time falls on a Saturday or Sunday.
     def on_weekend?
-      wday.in?(WEEKEND_DAYS)
+      WEEKEND_DAYS.include?(wday)
     end
 
     # Returns a new date/time the specified number of days ago.