install bson_ext with mongo gem to suppress warning

This will no longer appear when configuring the replicaset or shard:

```
[2014-01-28T14:36:43+00:00] INFO: service[mongodb] sending create action to ruby_block[config_replicaset] (delayed)
[2014-01-28T14:36:43+00:00] INFO: Processing ruby_block[config_replicaset] action create (mongodb::shard line 210)
      ** Notice: The native BSON extension was not loaded. **

      For optimal performance, use of the BSON extension is recommended.

      To enable the extension make sure ENV['BSON_EXT_DISABLED'] is not set
      and run the following command:

        gem install bson_ext

      If you continue to receive this message after installing, make sure that
      the bson_ext gem is in your load path.

[2014-01-28T14:36:43+00:00] INFO: Configuring replicaset with members shard1-mongodb01
[2014-01-28T14:36:43+00:00] INFO: ruby_block[config_replicaset] called
```