Use the latest boskos ✨

Using the same boskos version the prow folks are using, e.g.,
https://github.com/kubernetes/test-infra/blob/b2471685eed6a7d063d7e1e19032282bb33679db/prow/cluster/boskos.yaml#L65
which they bumped to in the context of dealing with the same issue we
ran into on Friday (#186)

boskos/boskos.yaml

@@ -89,7 +89,7 @@ spec:
       terminationGracePeriodSeconds: 30
       containers:
       - name: boskos
-        image: gcr.io/k8s-testimages/boskos:v20190621-ff01381
+        image: gcr.io/k8s-prow/boskos/boskos:v20200117-6384054e3
         args:
         - --storage=/store/boskos.json
         - --config=/etc/config/config
@@ -150,7 +150,7 @@ spec:
       terminationGracePeriodSeconds: 300
       containers:
       - name: boskos-janitor
-        image: gcr.io/k8s-testimages/janitor:v20190621-ff01381
+        image: gcr.io/k8s-prow/boskos/janitor:v20200117-6384054e3
         args:
         - --resource-type=gke-project
         - --pool-size=10
@@ -185,6 +185,6 @@ spec:
       terminationGracePeriodSeconds: 30
       containers:
       - name: boskos-reaper
-        image: gcr.io/k8s-testimages/reaper:v20190621-ff01381
+        image: gcr.io/k8s-prow/boskos/reaper:v20200117-6384054e3
         args:
         - --resource-type=gke-project