[ruby/forwardable] Update spec.files

https://github.com/ruby/forwardable/commit/1b6991e589

lib/forwardable/forwardable.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/ruby/forwardable"
   spec.license       = "BSD-2-Clause"
 
-  spec.files         = [".gitignore", ".travis.yml", "Gemfile", "LICENSE.txt", "README.md", "Rakefile", "bin/console", "bin/setup", "forwardable.gemspec", "lib/forwardable.rb", "lib/forwardable/impl.rb"]
+  spec.files         = ["forwardable.gemspec", "lib/forwardable.rb", "lib/forwardable/impl.rb", "lib/forwardable/version.rb"]
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]