修正 db:seed 错误

```
rake aborted!
uninitialized constant Setting
/Users/saberma/Documents/19wu/db/seeds.rb:10:in `<top (required)>'
/Users/saberma/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0/lib/rails/engine.rb:540:in `load_seed'
/Users/saberma/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/tasks/database_tasks.rb:153:in `load_seed'
/Users/saberma/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0/lib/active_record/railties/databases.rake:181:in `block (2 levels) in <top (required)>'
/Users/saberma/.rvm/gems/ruby-2.0.0-p247/bin/ruby_noexec_wrapper:14:in `eval'
/Users/saberma/.rvm/gems/ruby-2.0.0-p247/bin/ruby_noexec_wrapper:14:in `<main>'
Tasks: TOP => db:seed
```

#579