bump rubocop-rspec to 1.6
bump rubocop-rspec to 1.6

We already test with version 1.6 and updated our configs to reflect the
changes. There are a few new cops that could break on older
rubocop-rspec 1.5, so we should bump the requoired version. Initial 1.6
support came in https://github.com/voxpupuli/modulesync_config/pull/204

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