The driver has a DBRef class. Now that dbref support is partially implemented in bson-ruby (https://jira.mongodb.org/browse/RUBY-2087), this class should be moved to bson-ruby taking care to maintain compatibility in the driver for any of its users.

Copy DBRef driver class into bson-ruby
Verify all DBRef tests that are in driver were ported to bson-ruby
Verify driver test suite is green against bson master
Release bson-ruby 4.13.0
Bump driver dependency on bson-ruby to 4.13.0
Alias the driver's DBRef class to the bson-ruby one via constant assignment, remove existing Mongo::DBRef class from driver
Write release notes in driver explaining the move and giving an example of how to use the class (can be as simple as showing an example of creating Mongo::DBRef using existing driver versions and BSON::DBRef using the new bson-ruby)