Only build master branch, or PRs to it

Prevents appveyor from aggressively building feature branches and
incrementing the build number, independently of the separate PR build
that gets triggered anyway

Author: tathamoddie

appveyor.yml

@@ -1,6 +1,9 @@
 image: Visual Studio 2017
 
 skip_branch_with_pr: true
+branches:
+  only:
+    - master
 
 version: 2.1.0.{build}