FIX: remove nullable dates post upgrade to Rails 4

app/models/screened_email.rb

@@ -71,8 +71,8 @@ end
 #  action_type   :integer          not null
 #  match_count   :integer          default(0), not null
 #  last_match_at :datetime
-#  created_at    :datetime
-#  updated_at    :datetime
+#  created_at    :datetime         not null
+#  updated_at    :datetime         not null
 #  ip_address    :inet
 #
 # Indexes