Updated moped to 2.0, added compatibility for ruby_bson v1 and v2+

As discussed, updated moped to latest version.

Added compatibility for both `ruby_bson` v1 (currently used by `mongo`) and v2 (currently used by `moped`). Also made sure that if mongo updated to v2 in the future, things keep working like they should.

I've run the rspec tests for both official and moped. Moped succeeds with 0 failures. Official remains the same with 4 failures before and after my patch (mostly concurrency issues).