Use Dir#[] instead of git command for gemspec

Follow up rubocop/rubocop-performance@0b32b55.

Author: koic

rubocop-rails.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
   DESCRIPTION
 
   s.email = '[email protected]'
-  s.files = `git ls-files bin config lib LICENSE.txt README.md`.split($RS)
+  s.files = Dir['LICENSE.txt', 'README.md', 'config/**/*', 'lib/**/*']
   s.extra_rdoc_files = ['LICENSE.txt', 'README.md']
   s.homepage = 'https://github.com/rubocop/rubocop-rails'
   s.licenses = ['MIT']