(maint) switch to container-based travis builds
(maint) switch to container-based travis builds

Here we attempt to fixup the issue observed in recent travis-ci builds where
bundler is complaining that PATH contains /tmp/ (which is world-writable) by
switching to the newer container-based infrastructure.

MikaelSmith/marionette-collective,puppetlabs/marionette-collective,MikaelSmith/marionette-collective,shrug/marionette-collective,MikaelSmith/marionette-collective,puppetlabs/marionette-collective,shrug/marionette-collective,shrug/marionette-collective,shrug/marionette-collective,shrug/marionette-collective,puppetlabs/marionette-collective,puppetlabs/marionette-collective,puppetlabs/marionette-collective,MikaelSmith/marionette-collective,shrug/marionette-collective,puppetlabs/marionette-collective,MikaelSmith/marionette-collective,MikaelSmith/marionette-collective