add vs2017 RC to appveyor

Author: Kirk Shoop

appveyor.yml

@@ -6,11 +6,13 @@ branches:
         - master
         - appveyor
 
+image: Visual Studio 2017 RC
+
 environment:
   matrix:
     - VSVER: Visual Studio 12 2013 Win64
-    - os: Visual Studio 2015
-      VSVER: Visual Studio 14 2015 Win64
+    - VSVER: Visual Studio 14 2015 Win64
+    - VSVER: Visual Studio 15 2017 Win64
 
 platform:
   - x64
@@ -42,8 +44,9 @@ artifacts:
     type: zip
 
 notifications:
-  - provider: Webhook  
+  - provider: Webhook
     url: https://webhooks.gitter.im/e/4c53c094183ccaa8d059
+    method: POST
     on_build_success: true
     on_build_failure: true
     on_build_status_changed: false