Use Dir[] instead of git ls-files per rubocop-packaging advice

Author: jgarber623

indieweb-endpoints.gemspec

@@ -13,9 +13,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/indieweb/indieweb-endpoints-ruby'
   spec.license       = 'MIT'
 
-  spec.files         = Dir.chdir(File.expand_path(__dir__)) do
-    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(bin|spec)/}) }
-  end
+  spec.files         = Dir['lib/**/*', '*.md', '*.gemspec'].reject { |f| File.directory?(f) }
 
   spec.require_paths = ['lib']