Add missing dependency on moped

Here's an example stack trace:

```
/var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store/mongoid_store.rb:16:in `<class:Session>': uninitialized constant Moped::BSON (NameError)
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store/mongoid_store.rb:8:in `<class:MongoidStore>'
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store/mongoid_store.rb:6:in `<module:Session>'
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store/mongoid_store.rb:5:in `<module:ActionDispatch>'
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store/mongoid_store.rb:4:in `<top (required)>'
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store-rails3.rb:41:in `block in <top (required)>'
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store-rails3.rb:39:in `each'
        from /var/lib/gems/1.9.1/gems/mongo_session_store-rails3-4.1.1/lib/mongo_session_store-rails3.rb:39:in `<top (required)>'
```