Update rubocop requirement to ~> 0.51.0
Update rubocop requirement to ~> 0.51.0

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)
gocardless/activerecord-safer_migrations