4.13.0

This release of the BSON gem moves the Mongo::DBRef class to the bson-ruby repository as BSON::DBRef and enhances the functionality of DBRef:

• RUBY-2090 Move driver DBRef class to bson-ruby
• RUBY-2639 DBRef is now a subclass of Hash
• RUBY-2638 DBRef retains additional fields
• RUBY-2857 DBRef ensures $ref/$id/$db fields come first
• RUBY-2637 DBRef can no longer be created without $id

The following other improvements were made:

• RUBY-2763 Add BSON Binary subtype 7
• RUBY-2734 Prohibit embedded null bytes in strings used as BSON::Document hash keys, regexps and regexp options

This release of bson-ruby also increases the minimum Ruby version required to 2.5.