RUBY-805 use listCollections with wire version >= 3

I'm not sure what to do about the DB#collections_info API. In the past, it returned a cursor, that when iterated, would iterate over the collection info documents.

In executing the listCollections command in the method instead, I could return a cursor, but then iterating over it wouldn't iterate over the collection info documents. I've chosen instead to return the list "collections" in the command result. This is an API change, but the object return is still enumerable so it's not really a big deal.

This problem is manifested when #next_document is attempted on the result of DB#collections_info.
Should this be Mongo::MongoClient::MONGODB_2_8 to match the wire version bump?