Initialize MongoShardedClient with MONGODB_URI.

This fixes the errors that were occurring when instantiating a
MongoShardedClient when a MONGODB_URI environment variable is present.
This does not provide a means for a URIParser to determine whether or
not to instantiate a MongoSharedClient when asking for a connection,
since the information present in the URI cannot distinguish between
multiple mongos or a replica set.

[ RUBY-541 ]