Fix intermittent build failure due to timezones

Because our default timezone for the app is UTC, at certain times, some
tests could fail outside of UTC. This commit fixes that by changing a
single call to Time.now to Time.zone.now.

app/models/post.rb

@@ -9,7 +9,7 @@ class Post < ActiveRecord::Base
 
   def author_date_subject
     # slugs are created before created_at is set
-    time = created_at || Time.now
+    time = created_at || Time.zone.now
     "#{author.login_name} #{time.strftime("%Y%m%d")} #{subject}"
   end