Update appveyor.yml

Author: Dllieu

appveyor.yml

@@ -1,14 +1,14 @@
-version: "1.40.{build}"
+version: "1.50.{build}"
 
-#vc140
-os: Visual Studio 2015
+#vc150
+os: Visual Studio 2017
 
 configuration:
   - Debug
   - Release
 
 init:
-  - set PATH=C:\Program Files (x86)\MSBuild\14.0\Bin;%PATH%
+  - set PATH=C:\Program Files (x86)\MSBuild\15.0\Bin;%PATH%
   - cmd: msbuild /version
 
 clone_folder: C:\projects\VisualStudioCppExtensions