appveyor: always build the 'appveyor' branch

The 'appveyor' branch is useful for testing AppVeyor builds.
diff --git a/appveyor.yml b/appveyor.yml
index 9e94c07..fb3fff7 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -2,6 +2,7 @@
 branches:
   only:
   - master
+  - appveyor
   - /^maint.*/
 environment:
   GITTEST_INVASIVE_FS_STRUCTURE: 1