add testing on ruby2.4.0
add testing on ruby2.4.0

this is becomming the new Ruby version in the near future. The rolling
release distributions will soon update to it, so we should already test
against it. It is still under development, so we enable allow_failures
for it. Tested in https://github.com/voxpupuli/puppet-gluster/pull/60

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