Appveyor: Allow failing builds against newest toolset

appveyor.yml

@@ -1,5 +1,6 @@
 version: '{branch}.{build}'
 os: Windows Server 2012 R2
+
 configuration:
 - Debug
 - Release
@@ -9,11 +10,17 @@ environment:
   - PlatformToolset: v140
   - PlatformToolset: v120
   - PlatformToolset: Windows7.1SDK
+
 build_script:
 - >
   msbuild "json-c.vcxproj" /m /verbosity:normal
   /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
   /p:PlatformToolset=%PlatformToolset%
+
+matrix:
+  allow_failures:
+    - PlatformToolset: v140
+
 artifacts:
 - path: Debug\*
   name: Debug_x86