Add Ruby 2.3 to travis matrix
Add Ruby 2.3 to travis matrix

saghm/mongo-ruby-driver,kay-kim/mongo-ruby-driver,jonhyman/mongo-ruby-driver,saghm/mongo-ruby-driver,estolfo/mongo-ruby-driver,estolfo/mongo-ruby-driver,kay-kim/mongo-ruby-driver,mongodb/mongo-ruby-driver,wandenberg/mongo-ruby-driver,brandonblack/mongo-ruby-driver,jonhyman/mongo-ruby-driver,mongodb/mongo-ruby-driver,mongodb/mongo-ruby-driver