update CONTRIBUTING.md with version of go to download
update CONTRIBUTING.md with version of go to download

bryson/packer,paulmey/packer,dave2/packer,ricardclau/packer,pecigonzalo/packer,grubernaut/packer,tas50/packer,rnaveiras/packer,grubernaut/packer,rnaveiras/packer,pecigonzalo/packer,paulmey/packer,ricardclau/packer,bryson/packer,tas50/packer,grubernaut/packer,pecigonzalo/packer,hashicorp/packer,pecigonzalo/packer,tas50/packer,mitchellh/packer,grubernaut/packer,rnaveiras/packer,dave2/packer,timsutton/packer,hashicorp/packer,bryson/packer,hashicorp/packer,dave2/packer,paulmey/packer,hashicorp/packer,grubernaut/packer,paulmey/packer,timsutton/packer,dave2/packer,tas50/packer,grubernaut/packer,ricardclau/packer,mitchellh/packer,rnaveiras/packer,timsutton/packer,ricardclau/packer,timsutton/packer,dave2/packer,rnaveiras/packer,ricardclau/packer,ricardclau/packer,paulmey/packer,bryson/packer,tas50/packer,tas50/packer,bryson/packer,mitchellh/packer,mitchellh/packer,timsutton/packer,paulmey/packer,dave2/packer,rnaveiras/packer,bryson/packer,timsutton/packer,mitchellh/packer