Update rubocop requirement to ~> 0.56.0
Update rubocop requirement to ~> 0.56.0

Updates the requirements on [rubocop](https://github.com/bbatsov/rubocop) to permit the latest version.
- [Release notes](https://github.com/bbatsov/rubocop/releases)
- [Changelog](https://github.com/bbatsov/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bbatsov/rubocop/commits/v0.56.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
gocardless/prius