change what branches Appveyor builds

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300788 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/appveyor.yml b/appveyor.yml
index 6bd2ef1..3cb2ce6 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -10,7 +10,7 @@
 
 branches:
   only:
-    - windows-build
+    - master
 
 configuration:
   - Debug