Make Rubocop happy

I've noticed the build is broken because of some Rubocop violations:

* in the latest Rubocop version,  the `ReduceArguments` checker has been renamed to `SingleLineBlockParams`;
* a space before a closed brace was missing;
* the `has_key?` method has been deprecated in favor of `key?`.

This pull request fixes those problems. I've keps an alias for `has_key?`, so that nothing breaks.