Revert "appveyor.yml: don't use matrix."

This reverts commit d4ccd1e001.

appveyor.yml

@@ -3,20 +3,16 @@ environment:
   global:
     WIXBIN: "c:\\Program Files (x86)\\WiX Toolset v3.11\\bin"
     STACK_YAML: "c:\\pandoc\\stack.pkg.yaml"
-    STACK_VERSION: "windows-i386"
+  matrix:
+  - STACK_VERSION: "windows-i386"
     STACK_ROOT: "c:\\sr32"
     STACK: "%STACK_ROOT%\\stack.exe"
-
-# matrix:
-# - STACK_VERSION: "windows-i386"
-#   STACK_ROOT: "c:\\sr32"
-#   STACK: "%STACK_ROOT%\\stack.exe"
 #  - STACK_VERSION: "windows-x86_64"
 #    STACK_ROOT: "c:\\sr64"
 #    STACK: "%STACK_ROOT%\\stack.exe"
-#
-# matrix:
-#  fast_finish: true
+
+matrix:
+  fast_finish: true
 
 cache:
   - "%STACK_ROOT%"