travis: switch base OS from xenial to bionic
travis: switch base OS from xenial to bionic

This got tested at https://github.com/voxpupuli/puppet-collectd/pull/890

dhoppe/modulesync_config,voxpupuli/modulesync_config,voxpupuli/modulesync_config,dhoppe/modulesync_config,dhoppe/modulesync_config,voxpupuli/modulesync_config,dhoppe/modulesync_config,puppet-community/modulesync_config,voxpupuli/modulesync_config