Use latest skylighting.

This addresses #5776 for now, though ultimately we need a
new release of regex-pcre-builtin or some other solution.

pandoc.cabal

@@ -396,8 +396,8 @@ library
                  tagsoup >= 0.14.6 && < 0.15,
                  base64-bytestring >= 0.1 && < 1.1,
                  zlib >= 0.5 && < 0.7,
-                 skylighting >= 0.8.2 && < 0.9,
-                 skylighting-core >= 0.8.2 && < 0.9,
+                 skylighting >= 0.8.2.1 && < 0.9,
+                 skylighting-core >= 0.8.2.1 && < 0.9,
                  data-default >= 0.4 && < 0.8,
                  temporary >= 1.1 && < 1.4,
                  blaze-html >= 0.9 && < 0.10,

stack.yaml

@@ -16,6 +16,8 @@ extra-deps:
   commit: 860788f105cc9b3fec2ab062d993b7aebd68c9c0
 - pandoc-types-1.17.6.1
 - texmath-0.11.2.3
+- skylighting-0.8.2.1
+- skylighting-core-0.8.2.1
 - doclayout-0.1
 - HsYAML-0.2.0.0
 - HsYAML-aeson-0.2.0.0