github PRs: require branches to be up to date before merging (#111)

diff --git a/.asf.yaml b/.asf.yaml
index aa574b3..8bb9691 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -33,7 +33,7 @@
   protected_branches:
     master:
       required_status_checks:
-        strict: false
+        strict: true
         contexts:
           - "Travis CI - Pull Request"
       required_pull_request_reviews: