Update version to 4.0.0
Update version to 4.0.0

ukblewis/bson-ruby,mongodb/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,ukblewis/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,ukblewis/bson-ruby,ukblewis/bson-ruby,estolfo/bson-ruby