make wpscan ruby 2.4.0 compatible

fixes #1044

lib/common/common_helper.rb

@@ -96,7 +96,9 @@ end
 
 def update_required?
   date = last_update
-  (true if date.nil?) or (date < 5.days.ago)
+  day_seconds = 24 * 60 * 60
+  five_days_ago = Time.now - (5 * day_seconds)
+  (true if date.nil?) or (date < five_days_ago)
 end
 
 # Define colors

lib/common/hacks.rb

@@ -35,16 +35,3 @@ class Numeric
     s.sub(/\.?0*$/, ' ' + units[e])
   end
 end
-
-# time calculations
-class Fixnum
-  SECONDS_IN_DAY = 24 * 60 * 60
-
-  def days
-    self * SECONDS_IN_DAY
-  end
-
-  def ago
-    Time.now - self
-  end
-end