[Rails5] Fix ActionCable's mount_path configuration

The original MR [1] fixed red specs for Rails 5.
But while the failed specs were fixed, that changes brought new failed
specs which weren't caught up in [1].

This commit just fixes the fix. :)

New errors are like these:

```
Failed to upgrade to WebSocket (REQUEST_METHOD: GET, HTTP_CONNECTION: keep-alive, HTTP_UPGRADE: )
Finished "/-/boards/1/lists"[non-WebSocket] for 127.0.0.1 at 2018-06-20 18:09:26 +0200
```

[1]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20015

config/application.rb

@@ -62,7 +62,7 @@ module Gitlab
     # namespaces/users.
     # https://github.com/rails/rails/blob/5-0-stable/actioncable/lib/action_cable.rb#L38
     # Please change this value when configuring ActionCable for real usage.
-    config.action_cable.mount_path = "-" if rails5?
+    config.action_cable.mount_path = "/-/cable" if rails5?
 
     # Configure sensitive parameters which will be filtered from the log file.
     #