Only build master for AppVeyor

We only have one AppVeyor worker and building topic branches causes us
to build large backups in our queues. This is made worse when folks
are iterating on a PR because each push queues not only a PR
build (which we want) but also the topic branch build (which we don't
care about so much).

.appveyor.yml

@@ -3,6 +3,9 @@ image: Visual Studio 2017
 configuration: Release
 platform: x64
 clone_folder: '%USERPROFILE%\go\src\github.com\pulumi\pulumi'
+branches:
+    only:
+        - master
 init:
 - ps: Install-Product node 6.10.2 x64
 environment: