AppVeyor: build only from master

We want to do official builds from master only.
PR builds should be not affected by this.

Author: ge0rdi

appveyor.yml

@@ -1,4 +1,7 @@
 version: 4.4.{build}
+branches:
+  only:
+    - master
 pull_requests:
   do_not_increment_build_number: true
 skip_tags: true