Require texmath 0.8.6.2.

Closes several texmath-related bugs:  #2775, #2310, #2310.

This fixes issues with sub/superscript positioning and
matrix column alignment in docx.

pandoc.cabal

@@ -257,7 +257,7 @@ Library
                  text >= 0.11 && < 1.3,
                  zip-archive >= 0.2.3.4 && < 0.4,
                  HTTP >= 4000.0.5 && < 4000.4,
-                 texmath >= 0.8.6.1 && < 0.9,
+                 texmath >= 0.8.6.2 && < 0.9,
                  xml >= 1.3.12 && < 1.4,
                  random >= 1 && < 1.2,
                  extensible-exceptions >= 0.1 && < 0.2,

stack.yaml

@@ -8,7 +8,7 @@ flags:
 packages:
 - '.'
 extra-deps:
-- texmath-0.8.6.1
+- texmath-0.8.6.2
 - data-default-0.6.0
 - data-default-instances-base-0.1.0
 # to compile against aeson 0.11.0.0: