travis: add ruby 2.6.0
travis: add ruby 2.6.0

ramiroaraujo/image_optim,mihailim/image_optim,ramiroaraujo/image_optim,ramiroaraujo/image_optim,toy/image_optim,mihailim/image_optim,toy/image_optim,toy/image_optim,mihailim/image_optim