Limit GH pull request to rebases

 patch by Mick Semb Wever; reviewed by Benjamin Lerer, Berenguer Blasi, Brandon Williams, Ekaterina Dimitrova, Stefan Miklosovic for CASSANDRA-17419

.asf.yml

@@ -2,4 +2,4 @@ github:
   enabled_merge_buttons:
     squash:  false
     merge:   false
-    rebase:  false
+    rebase:  true