update stack.yaml.lock, Checklist

Checklist

@@ -25,7 +25,6 @@ Before release:
     stack.yaml
     fullcheck
     fullcheck.ps1
-    (not necessarily benchmark/stack.yaml)
     .travis.yml (if present)
 
 6. Use latest stack

stack.yaml.lock

@@ -49,7 +49,7 @@ packages:
     commit: d240eb7334cd08c7e377b59fcb1977bdf95527d6
 snapshots:
 - completed:
-    size: 532172
-    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/16/18.yaml
-    sha256: 4f2a092c6f4869854e8d7435ab98ce5157c641022c3cbfc4c4614ff3db752e62
-  original: lts-16.18
+    size: 532413
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/16/21.yaml
+    sha256: 7b5cac89352fa4a88606dc6cb250aee9291f21e2e988d464065f5aa51f5de33d
+  original: lts-16.21