Start testing on ruby 2.3.0 on travis
Start testing on ruby 2.3.0 on travis

Also update ruby 2.1 and 2.2 to 2.1.8 and 2.2.4.

celsworth/roda,jeremyevans/roda,jeremyevans/roda,celsworth/roda,celsworth/roda,jeremyevans/roda