disable AppVeyor for branches other than master

This should disable AppVeyor from running against dev-v3 PRs.

Signed-off-by: Matthew Fisher <matt.fisher@microsoft.com>

.appveyor.yml

 version: "{build}"
+branches:
+  only:
+  - master
 clone_folder: c:\go\src\k8s.io\helm
 environment:
   GOPATH: c:\go