Use symbols instead of strings to define the keys of the mongo auths option

It seems mongo-ruby-driver v1.8.4 switched from strings to symbols when referencing the MongoClient::@auths hash

https://github.com/mongodb/mongo-ruby-driver/commit/6428bf699697dbd93622e0a411b5b153295bfe4e#L1L244
Fixed mongo_spec to work with mongo >= 1.8.4
Updated mongo backend to work with mongo > 1.8.3

Starting in Mongo 1.8.4 the keys of the auths hash are symbols. The qu-mongo backend probably shouldn't care how mongo keys its hashes so I updated the code to avoid manipulating auths directly.

Also, the Connection class appears to be deprecated so I updated this to use the replacement MongoClient instead.
Merge pull request #65 from joshfreed/master

Fix for issue 64 and mongo > 1.8.3