change to a working cygwin repo

appveyor.yml

@@ -9,7 +9,7 @@ environment:
         CYG_ARCH: x86
         CYG_ROOT: C:/cygwin
         CYG_SETUP_URL: http://cygwin.com/setup-x86.exe
-        CYG_MIRROR: http://cygwin.mirror.constant.com
+        CYG_MIRROR: http://mirrors.kernel.org/sourceware/cygwin/
         CYG_CACHE: C:\cygwin\var\cache\setup
         CYG_BASH: C:/cygwin/bin/bash