Use latest texmath.

pandoc.cabal

@@ -393,7 +393,7 @@ library
                  safe >= 0.3 && < 0.4,
                  zip-archive >= 0.2.3.4 && < 0.5,
                  HTTP >= 4000.0.5 && < 4000.4,
-                 texmath >= 0.12 && < 0.13,
+                 texmath >= 0.12.0.1 && < 0.13,
                  xml >= 1.3.12 && < 1.4,
                  split >= 0.2 && < 0.3,
                  random >= 1 && < 1.2,

stack.yaml

@@ -13,7 +13,7 @@ packages:
 - '.'
 extra-deps:
 - pandoc-types-1.20
-- texmath-0.12
+- texmath-0.12.0.1
 - haddock-library-1.8.0
 - skylighting-0.8.3.2
 - skylighting-core-0.8.3.2