[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)

.rubocop.yml

@@ -102,6 +102,10 @@ Rails/Exit:
 Rails/HasAndBelongsToMany:
   Enabled: false
 
+# FIXME: Disabled due to a bug in Rubocop. Once it is fixed in this PR (https://github.com/bbatsov/rubocop/pull/5707), it could be enabled again
+Rails/HttpStatus:
+  Enabled: false
+
 # Checks for the use of output calls like puts and print
 Rails/Output:
   Exclude: