Removing Base::GroupedCollections from SelectInput

There is no GroupedCollections module in 3.0-stable branch of Formtastic gem (https://github.com/justinfrench/formtastic/tree/3.0-stable/lib/formtastic/inputs/base)

I get "Unable to find input class for select" when I try to create:
```ruby
input :position, as: :select, collection: @positions
```