Update redis-rack to 2.0.4

Up until version 2.0.3 redis-rack included a "rake" binary that would
overwrite/hijack the one provided by Rake itself. Unfortunately the
binary provided by redis-rack would produce errors in many cases. See
https://github.com/redis-store/redis-rack/pull/34 for more info.

Gemfile.lock

@@ -718,7 +718,7 @@ GEM
       redis-store (>= 1.3, < 2)
     redis-namespace (1.5.2)
       redis (~> 3.0, >= 3.0.4)
-    redis-rack (2.0.3)
+    redis-rack (2.0.4)
       rack (>= 1.5, < 3)
       redis-store (>= 1.2, < 2)
     redis-rails (5.0.2)

changelogs/unreleased/update-redis-rack.yml

+---
+title: Update redis-rack to 2.0.4
+merge_request:
+author:
+type: other