Fix the redis URL (#463)

We upgraded the redis environment on Heroku a couple days ago. In the new environment, the REDIS_URL environment variable was changed to HEROKU_REDIS_MAROON_TLS_URL.
Updated the code where we were hardcoding it to the old to url.

Fixes #462

Author: Mariatta

miss_islington/tasks.py

@@ -16,7 +16,8 @@
 app = celery.Celery("backport_cpython")
 
 app.conf.update(
-    BROKER_URL=os.environ["REDIS_URL"], CELERY_RESULT_BACKEND=os.environ["REDIS_URL"]
+    BROKER_URL=os.environ["HEROKU_REDIS_MAROON_TLS_URL"],
+    CELERY_RESULT_BACKEND=os.environ["HEROKU_REDIS_MAROON_TLS_URL"],
 )
 
 cache = cachetools.LRUCache(maxsize=500)

tests/test_backport_pr.py

@@ -7,7 +7,7 @@
 import redis
 import kombu
 
-os.environ["REDIS_URL"] = "someurl"
+os.environ["HEROKU_REDIS_MAROON_TLS_URL"] = "someurl"
 
 from miss_islington import backport_pr