Exclude unnecessary files from gem packages

[ksss]

Exclude development directories such as bin and .github from gem packages.

Before

[".github/dependabot.yml",
 ".github/workflows/push_gem.yml",
 ".github/workflows/test.yml",
 ".gitignore",
 "BSDL",
 "COPYING",
 "Gemfile",
 "README.md",
 "Rakefile",
 "benchmark.gemspec",
 "bin/console",
 "bin/setup",
 "lib/benchmark.rb"]

After

[".gitignore",
 "BSDL",
 "COPYING",
 "Gemfile",
 "README.md",
 "Rakefile",
 "benchmark.gemspec",
 "lib/benchmark.rb"]