Fix a bug with mongo-ruby-driver 1.8 compatibility.
Fix a bug with mongo-ruby-driver 1.8 compatibility.

We can accept either a Mongo::MongoClient or a
Mongo::Connection. Since we can't guarantee Mongo::MongoClient is
defined (since we support m-r-d 1.7), just check for a 'db' method,
for now.

stripe/mongoriver,johnleetran/mongoriver