Reduce gem size

[ybiquitous]

The current published gem includes test files or build files that are actually unneeded.
This change removes such files from a published gem.

In addition, this removes test_files from the gemspec since it is no longer supported.
See https://guides.rubygems.org/specification-reference/

Verification:

$ bundle exec rake build && gem unpack pkg/*.gem
public_suffix 5.0.3 built to pkg/public_suffix-5.0.3.gem.
Unpacked gem: '/Users/masafumi.koba/git/ybiquitous/publicsuffix-ruby/public_suffix-5.0.3'

$ tree public_suffix-*
public_suffix-5.0.3/
├── .yardopts
├── 2.0-Upgrade.md
├── CHANGELOG.md
├── LICENSE.txt
├── README.md
├── SECURITY.md
├── data/
│   └── list.txt
└── lib/
    ├── public_suffix/
    │   ├── domain.rb
    │   ├── errors.rb
    │   ├── list.rb
    │   ├── rule.rb
    │   └── version.rb
    └── public_suffix.rb

4 directories, 13 files

[weppos]

Thanks!