Reduce gem size by excluding test files

[yuri-zubov]

This pull request updates the *.gemspec file to optimize the gem package size and structure

$ gem build -o before.tar.gz

$ git switch reduce-gem-size

$ gem build -o after.tar.gz

$ du -sh before.tar.gz after.tar.gz
 24K	before.tar.gz
 16K	after.tar.gz
 

	before	after	saved
size	24K	16K	-8K (⏷ -33.33%)

whitch files was deleted?

 data
-├── .document
-├── .gitignore
-├── .travis.yml
-├── Gemfile
 ├── lib
 │   ├── webrobots
 │   │   ├── nokogiri.rb
 │   │   ├── robotstxt.rb
 │   │   ├── robotstxt.ry
 │   │   └── version.rb
 │   └── webrobots.rb
 ├── LICENSE.txt
-├── Rakefile
 ├── README.rdoc
-├── test
-│   ├── helper.rb
-│   ├── simplecov_start.rb
-│   ├── test_webrobots.rb
-│   └── vcr_cassettes
-│       └── major_sites.yml
-└── webrobots.gemspec