Disable Appveyor builds on branches.

Our account for Appveyor allows only 1 concurrent build.  Our current Appveyor config builds every commit to a PR twice -- once as the PR and once as the branch.  This CL updates our Appveyor config to disable builds on branches, so we get only one build per PR commit.

PiperOrigin-RevId: 320734870

Author: Eddie Aftandilian

appveyor.yml

@@ -1,4 +1,10 @@
+# Don't build branches that are not PRs, to avoid double builds.
+branches:
+  only:
+    - master
+
 os: Visual Studio 2015
+
 install:
   - ps: |
       Add-Type -AssemblyName System.IO.Compression.FileSystem