Store boolean values in Redis as valid JSON

Calling `.set()` with `True` stores the byte string `'True'` which
cannot subsequently be decoded from JSON (because boolean values in
JSON are lowercase, ie `true`).

app/notify_client/job_api_client.py

@@ -90,7 +90,7 @@ class JobApiClient(NotifyAdminAPIClient):
 
         self.redis_client.set(
             'has_jobs-{}'.format(service_id),
-            True,
+            b'true',
             ex=cache.TTL,
         )