[ci] Disable rubocop cop Rails/HttpStatus

It is causing problems due to a bug:
```
An error occurred while Rails/HttpStatus cop was inspecting
/home/travis/build/openSUSE/open-build-service/src/api/app/controllers/application_controller.rb:350:4.
uninitialized constant Rack
Did you mean?  Racc
```

It has been reported already
[here](https://github.com/bbatsov/rubocop/issues/5745). To fix it, we
either disable the cop or put the gem `rack` explicitly in the
`Gemfile`. I prefer to disable the cop as it doesn't bring value in my
opinion. See its
[description](https://github.com/bbatsov/rubocop/blob/master/manual/cops_rails.md#railshttpstatus)