Only publish to Google Play if master branch (#848)

appveyor.yml

@@ -110,7 +110,7 @@ on_success:
       npm run deploy
     fi
 - ps: |
-    if($isWindows -and $env:PLAY_DEC_SECRET) {
+    if($isWindows -and $env:PLAY_DEC_SECRET -and $env:APPVEYOR_REPO_BRANCH -eq 'master') {
       secure-file\tools\secure-file -decrypt store\google\Publisher\play_creds.json.enc -secret $env:PLAY_DEC_SECRET
       cd store\google\Publisher\bin\Release\netcoreapp2.0
       dotnet Publisher.dll `