We made a mistake in opening up Kibana scripted fields to any and all
langs enabled in Elasticsearch. Kibana needs to do some magic to get
certain scripted field features to work
(https://github.com/elastic/kibana/pull/9171) and we can't commit to
supporting anything beyond expression and painless scripts. On top of
that, the other languages are being removed in ES 6.0 anyway. This commit
makes that stance explicit by disallowing anything other than those two
choices.