DataMapper 1.2 still needs Rails 3.2...
Fix the test suite for mongoid 40

It didn't run due to:

  moped-2.0.2/lib/moped/cluster.rb:254:in `with_primary':
    Could not connect to a primary node for replica set #<Moped::Cluster @seeds=[<Moped::Node resolved_address="127.0.1.1:27017">]>
    (Moped::Errors::ConnectionFailure)
Fix mongo mapper test suite

https://travis-ci.org/amatsuda/kaminari/jobs/41827391#L98