Enable AppVeyor webhook for Gitter

appveyor.yml

@@ -1,11 +1,19 @@
 version: 1.0.{build}-{branch}
 image:
-- Visual Studio 2017
-- Visual Studio 2017 Preview
-- Visual Studio 2015
-- Visual Studio 2013
+  - Visual Studio 2017
+  - Visual Studio 2017 Preview
+  - Visual Studio 2015
+  - Visual Studio 2013
 
 install:
-- ps: Install-Product node 8.9.1 x64
+  - ps: Install-Product node 8.9.1 x64
 build_script:
-- pwsh: ./scripts/ci.ps1
+  - pwsh: ./scripts/ci.ps1
+
+notifications:
+  - provider: Webhook
+    url: https://webhooks.gitter.im/e/7a2944eb6940f46466b9
+    method: POST
+    on_build_success: true
+    on_build_failure: true
+    on_build_status_changed: true