Disable Style/VariableNumber rubocup
Disable Style/VariableNumber rubocup

Linter was complaining about:

```
Style/VariableNumber: Use normalcase for variable numbers.
    @taxon_3 = content_item_with_details(
```

See http://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/VariableNumber

It didn't seem worth fixing this as the rest of the file uses this notation as well. We've already disabled this cop in [govuk_taxonomy_helpers](https://github.com/alphagov/govuk_taxonomy_helpers/blob/1fb4e1d1d952395f25b77c6f37da379f1ebe0657/.rubocop.yml#L8-L9), so we do the same thing here.

alphagov/content-tagger,alphagov/content-tagger,alphagov/content-tagger