Remove development files from the gem package

[orien]

There are several files in the gem package that aren't useful for
downstream projects. Removing these files reduces the gem package
size from 22K to 19K.

gem contents diff

< .buildkite/pipeline.yml
< .buildkite/steps/rspec.sh
< .buildkite/steps/rubocop.sh
< .github/workflows/ci.yml
< .gitignore
< .rspec
< .rubocop.yml
  CHANGELOG.md
< CODE_OF_CONDUCT.md
< Gemfile
< Guardfile
  LICENSE.txt
  README.md
< Rakefile
< bin/console
< bin/setup
  exe/unwrappr
  lib/unwrappr.rb
  lib/unwrappr/bundler_command_runner.rb
  lib/unwrappr/cli.rb
  lib/unwrappr/gem_change.rb
  lib/unwrappr/gem_version.rb
  lib/unwrappr/git_command_runner.rb
  lib/unwrappr/github/client.rb
  lib/unwrappr/github/pr_sink.rb
  lib/unwrappr/github/pr_source.rb
  lib/unwrappr/lock_file_annotator.rb
  lib/unwrappr/lock_file_comparator.rb
  lib/unwrappr/lock_file_diff.rb
  lib/unwrappr/octokit.rb
  lib/unwrappr/researchers/composite.rb
  lib/unwrappr/researchers/github_comparison.rb
  lib/unwrappr/researchers/github_repo.rb
  lib/unwrappr/researchers/ruby_gems_info.rb
  lib/unwrappr/researchers/security_vulnerabilities.rb
  lib/unwrappr/ruby_gems.rb
  lib/unwrappr/spec_version_comparator.rb
  lib/unwrappr/version.rb
  lib/unwrappr/writers/composite.rb
  lib/unwrappr/writers/github_commit_log.rb
  lib/unwrappr/writers/project_links.rb
  lib/unwrappr/writers/security_vulnerabilities.rb
  lib/unwrappr/writers/title.rb
  lib/unwrappr/writers/version_change.rb
< unwrappr.gemspec