Fix cache path (#685)

appveyor.yml

@@ -11,7 +11,7 @@ environment:
         CYG_ROOT: C:/cygwin
         CYG_SETUP_URL: http://cygwin.com/setup-x86.exe
         CYG_MIRROR: http://cygwin.mirror.constant.com
-        CYG_CACHE: C:/cygwin/var/cache/setup
+        CYG_CACHE: C:\cygwin\var\cache\setup
         CYG_BASH: C:/cygwin/bin/bash
 
 cache: