DEV: Only warn about invalid email setting in production

In development and test mode you don't care about email domains. Now
that `rake db:migrate` outputs a lot less this felt like an error when
it's really a warning.

db/fixtures/990_settings.rb

@@ -5,6 +5,8 @@ if SiteSetting.notification_email == SiteSetting.defaults[:notification_email]
   begin
     SiteSetting.notification_email = "noreply@#{Discourse.current_hostname}"
   rescue Discourse::InvalidParameters
-    STDERR.puts "Discourse hostname: #{Discourse.current_hostname} is not a valid domain for emails!"
+    if Rails.env.production?
+      STDERR.puts "WARNING: Discourse hostname: #{Discourse.current_hostname} is not a valid domain for emails!"
+    end
   end
 end