Require jquery/date_picker_bridge unless date_picker bridge was excluded

Fixed error when exclude date_picker bridge.

```ruby
  # initializer
  ActiveScaffold.exclude_bridges = [:date_picker]
```

Error
```
  undefined method `localization' for nil:NilClass (in .../active_scaffold-505e819b79ec/app/assets/javascripts/jquery/date_picker_bridge.js.erb)
```