Add rubocop to ci

[olleolleolle]

This adds a bare-bones RuboCop configuration. Fixes #927.

• Enable only Security checks.
• Make all fails that now exist "accepted as TODOs" and have them in the file (See https://docs.rubocop.org/rubocop/usage/basic_usage.html#command-line-flags for the --regenerate-todo option explanation and workflow).
• Add a CI step for this.
• Set it to care about Ruby 2.5 and up syntax.

[minad]

Thanks. This is a good start. I'd like to check if we can enable more rules than just the security rules.

[olleolleolle]

https://docs.rubocop.org/rubocop/cops.html lists all the ones built in.