[settings] Use json serializer for celery

vrobbler/settings.py

@@ -105,6 +105,8 @@ CELERY_BROKER_URL = REDIS_URL if REDIS_URL else "memory://localhost/"
 CELERY_RESULT_BACKEND = "django-db"
 CELERY_TIMEZONE = os.getenv("VROBBLER_TIME_ZONE", "America/New_York")
 CELERY_TASK_TRACK_STARTED = True
+CELERY_TASK_SERIALIZER = "json"
+CELERY_ACCEPT_CONTENT = ["json"]
 
 CELERY_BEAT_SCHEDULE = {
     "build-yesterdays-charts": {