bump stackage version

Author: robx

stack.yaml

@@ -3,4 +3,4 @@ ghc-options:
   puzzle-draw: -Werror
 packages:
 - '.'
-resolver: lts-15.0
+resolver: lts-15.8

stack.yaml.lock

@@ -0,0 +1,12 @@
+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/lock_files
+
+packages: []
+snapshots:
+- completed:
+    size: 492015
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/15/8.yaml
+    sha256: 926bc3d70249dd0ba05277ff00943c0addb35b627cb641752669e7cf771310d0
+  original: lts-15.8