[build] only build master on Appveyor so PRs don't trigger multiple builds

.appveyor.yml

@@ -3,6 +3,10 @@ version: 1.0.{build}
 image: Visual Studio 2015
 platform: x64
 
+branches:
+  only:
+    - master
+
 environment:
   matrix:
   - COMPILER: msys2