Add `Style/` prefix for consistency
Update Rubocop version to fix warning

The current locked version of Rubocop doesn't recognize the `Style/`
prefix for the config keys and generates the following warning:
```
Warning: unrecognized cop Style/CollectionMethods found in
/home/su/code/Markus/.rubocop.yml
```

Hence the update.
Merge pull request #1683 from zhangsu/rubocop_fix

Update Rubocop version to fix warning