[Rails5] Fix ActionCable's mount_path configuration
[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

jirutka/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,iiet/iiet-git,mmkassem/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,axilleas/gitlabhq,dreampet/gitlab,dreampet/gitlab,mmkassem/gitlabhq