Update version to 4.2.0.rc0
Update version to 4.2.0.rc0

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