Update prometheus and alertmanager to recent releases

Author: beorn7

prometheus-ksonnet/images.libsonnet

@@ -1,10 +1,10 @@
 {
   _images+:: {
-    prometheus: 'prom/prometheus:v2.18.1',
+    prometheus: 'prom/prometheus:v2.19.1',
     grafana: 'grafana/grafana:6.4.4',
     watch: 'weaveworks/watch:master-5b2a6e5',
     kubeStateMetrics: 'gcr.io/google_containers/kube-state-metrics:v1.6.0',
-    alertmanager: 'prom/alertmanager:v0.20.0',
+    alertmanager: 'prom/alertmanager:v0.21.0',
     nodeExporter: 'prom/node-exporter:v0.18.1',
     nginx: 'nginx:1.15.1-alpine',
   },