Update gemspec file to latest style standards

[epistrephein]

Pull Request description

• Remove test_files attribute since it's deprecated and no longer used.
• Use Dir[] instead of git ls-files to specify gem files, in order to improve readability and exclude unnecessary files.
• Don't include development files and RSpec tests in the built gem.

Pull Request checklist

Kind of change

• My code is a minor change (typo, configuration, anything not related to core functionality)
• My code is a bug fix (non-breaking change which fixes an issue)
• My code is a new feature (non-breaking change which adds functionality)
• My code introduces a breaking change (fix or feature that would break existing functionality)

Style and tests

• My code follows the code style of this project
• I have added tests to cover my changes
• All new and existing tests passed

Documentation

• My change requires a change to the documentation
• I have updated the documentation accordingly

Changelog

• I have included the CHANGELOG entry for the changes