Updated stemcell to version 170.13

Author: ci bot

buildstack.yml

@@ -116,7 +116,7 @@ releases:
 stemcells:
 - alias: default
   os: ubuntu-xenial
-  version: "170.12"
+  version: "170.13"
 update:
   canaries: 1
   canary_watch_time: 15000-300000