Add John Hixson to contributors

Maintainer of: audio/pianobar
audio/pithos
net/libstorj
net/nsq
net/py-libstorj
security/ngrok
sysutils/consul-alerts
sysutils/ipfs-go
sysutils/mtpfs
sysutils/nomad
sysutils/py-consul
sysutils/py-nomad
www/grafana3
www/minio-client
www/py-django-json-rpc
www/py-django-simple-captcha
www/py-djangotoolbox

PR: 225565