Use skylighting 0.10.4.

This version of skylighting uses xml-conduit rather than hxt.
This speeds up parsing of XML syntax definitions fourfold, and
removes four packages from pandoc's dependency graph:

hxt-charproperties
hxt-unicode
hxt-regex-xmlschema
hxt

pandoc.cabal

@@ -480,8 +480,8 @@ library
                  random                >= 1        && < 1.2,
                  safe                  >= 0.3      && < 0.4,
                  scientific            >= 0.3      && < 0.4,
-                 skylighting           >= 0.10.3   && < 0.11,
-                 skylighting-core      >= 0.10.3   && < 0.11,
+                 skylighting           >= 0.10.4   && < 0.11,
+                 skylighting-core      >= 0.10.4   && < 0.11,
                  split                 >= 0.2      && < 0.3,
                  syb                   >= 0.1      && < 0.8,
                  tagsoup               >= 0.14.6   && < 0.15,

stack.yaml

@@ -13,8 +13,8 @@ extra-deps:
 - commonmark-pandoc-0.2.0.1
 - tasty-bench-0.2.1
 - citeproc-0.3.0.7
-- skylighting-0.10.3
-- skylighting-core-0.10.3
+- skylighting-0.10.4
+- skylighting-core-0.10.4
 #- citeproc:
 #  git: https://github.com/jgm/citeproc.git
 #  commit: feb3b7580c6738eec3b23921f7c1739cfba611aa