Work around a splat assignment bug in Rubinius

[The current build](https://travis-ci.org/bbatsov/rubocop/jobs/28376409) is failing due to a bug in Rubinius.

This splat assignment fails on Rubinius 2.2.9:

```ruby
b.freeze
*_conditions, body = *b
```

```
  RuntimeError:
    can't modify frozen instance of Array
  # kernel/common/array.rb:988:in `pop'
  # ./lib/rubocop/cop/style/indentation_width.rb:99:in `on_case'
```

It seems to be fixed on the current Rubinius's master (0a92c3c).