README: update URL to unit test badge
Due to a Jenkins upgrade (I think?) the URL for the unit test badge
changed. Fix this.
# etcd operator
unit/integration:
-[![Build Status](https://jenkins-etcd-public.prod.coreos.systems/view/operator/job/etcd-operator-unit-master/badge/icon)](https://jenkins-etcd-public.prod.coreos.systems/view/operator/job/etcd-operator-unit-master/)
+[![Build Status](https://jenkins-etcd-public.prod.coreos.systems/job/etcd-operator-unit-master/badge/icon)](https://jenkins-etcd-public.prod.coreos.systems/job/etcd-operator-unit-master/lastBuild/)
e2e (Kubernetes stable):
[![Build Status](https://jenkins-etcd-public.prod.coreos.systems/buildStatus/icon?job=etcd-operator-master)](https://jenkins-etcd-public.prod.coreos.systems/job/etcd-operator-master/)
e2e (upgrade):
