contrib/kube-prometheus: Bump the prometheus-operator jsonnet dep
contrib/kube-prometheus: Bump the prometheus-operator jsonnet dep

kube-prometheus now contains a script, sync-to-internal-registry.jsonnet, that
depends on commit 8c6a68760010347134e41f3aa3d73c68eb094a1b to work. Bump the
prometheus-operator hash accordingly.

jescarri/prometheus-operator,coreos/prometheus-operator,coreos/prometheus-operator,jescarri/prometheus-operator