Reduce system of `EnsembleVertex` system region to 8 words.
Reduce system of `EnsembleVertex` system region to 8 words.

ctn-archive/nengo_spinnaker_2014,ctn-archive/nengo_spinnaker_2014,ctn-archive/nengo_spinnaker_2014,ctn-archive/nengo_spinnaker_2014