remove version locking

Currently if you install mongode, npm will download mongodb@0.9.9-3 as a dependency.

mongodb@0.9.9-3 was released almost a year ago, and there was a lot of changes there, see [HISTORY](https://github.com/mongodb/node-mongodb-native/blob/master/HISTORY). So mongode users won't get these changes by default.

I tested mongode with recent mongodb version, and it works fine, so I don't see a reason to depend on that exact version.