Relation#all is deprecated in Rails 4, use Relation#to_a instead

When I use simple_form with association, got a warning :

```
DEPRECATION WARNING: Relation#all is deprecated. If you want to eager-load a relation, you can call #load (e.g. `Post.where(published: true).load`). If you want to get an array of records from a relation, you can call #to_a (e.g. `Post.where(published: true).to_a`).
```

so maybe it's a good time to make this small adjustment ?