Updates to latest Rubocop
Updates `.rubocop.yml` for newer syntax
Removes useless `private` access modifiers

The call to `private` does nothing here:

```ruby
class Whatever
  private

  def self.method_name # <- will not be private, due to `self.`
    # ...
  end
end
```
Fixes Rubocop trailing comma preferences

Updated to match web app style.
Fixes Rubocop violations

- Added preference to prefer `%r{pathname/}` over `/pathname\//`
- Prefixed unused parameter names with underscores
- Updated `gemspec` file listing to match newer style