ci: ignore the failure when trying to kill redis

.drone.yml

@@ -5,7 +5,7 @@ name: default
 steps:
 - name: Run Tests and Generate Coverage Report
   commands:
-  - kill $(ps aux | grep 'redis-server 127.0.0.1:6389' | awk '{print $2}')
+  - kill $(ps aux | grep 'redis-server 127.0.0.1:6389' | awk '{print $2}') || true
   - redis-server --bind 127.0.0.1 --port 6389 >/dev/null &
   - coverage run -m pytest -q
   - coverage xml