Add a mechanism for invalidating that AppVeyor cache

appveyor.yml

@@ -3,12 +3,10 @@ version: '{build}'
 environment:
   # Keep the path as short as possible, just in case.
   STACK_ROOT: c:\s
-  # Appveyor does not seem to be able to cope with the symbolic link
-  # stack.yaml, so this is a workaround.
   RELEASE_USER: purescript
   RELEASE_REPO: purescript
 cache:
-- c:\s
+- c:\s -> appveyor/cache-buster.txt
 install:
 - git submodule update --init
 - ps: Install-Product node 5

appveyor/cache-buster.txt

@@ -0,0 +1,8 @@
+This file acts as a cache buster for the AppVeyor (Windows CI) build
+cache. In order to invalidate the AppVeyor build cache, simply make a
+change to the number at the end of this file and commit.
+
+See http://www.appveyor.com/docs/build-cache#cache-dependencies for more
+information.
+
+Increment me to invalidate the cache: 0