.travis.yml: allow arm64-linux to fail

Ditto for a833eb29f7

.travis.yml

@@ -458,6 +458,7 @@ matrix:
     - name: -fsanitize=address
     - name: -fsanitize=memory
     - name: arm32-linux
+    - name: arm64-linux
   fast_finish: true
 
 before_script: