Don't require dm-aggregates in will_paginate/data_mapper

Not all DM adapters implement the necessary interface for the dm-aggregates plugin, but may instead implement a #count method by other means. Since will_paginate really only needs a working #count method on the DM collection to function, let DM users require dm-aggregates in their own code if they want it.

I've also added a require of dm-aggregates to the DM test connector to maintain the present behavior of that test.