[AppVeyor] use the default image

to allow faster builds when the update is available: http://www.appveyor.com/blog/2015/06/23/new-oss-build-environment-and-xamarin-support

appveyor.yml

@@ -1,7 +1,5 @@
 version: "{build}"
 
-os: unstable #http://help.appveyor.com/discussions/suggestions/427-pre-install-cygwin
-
 platform:
     - Win32