Support new write procolol in mongo 2.6

[Duy Le Duc]

Since gen10's stable mongo driver doesn't compatible with bson 2.0, we can't use mongoid 4 with gen10's driver anymore. 

If we want to use new write protocol introduced in mongo 2.6 (with bulk support), there's no way to do it now.

Just want to brought it up :)

http://docs.mongodb.org/manual/release-notes/2.6/#new-write-operation-protocol

https://github.com/mongodb/mongo-ruby-driver/wiki/Bulk-Write-Operations