[ruby/tmpdir] Update gemspec according to the recent bundler template

ruby/tmpdir@4ef7bc4f2a

lib/tmpdir.gemspec

@@ -15,8 +15,11 @@ Gem::Specification.new do |spec|
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
-  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
-    `git ls-files -z 2>#{IO::NULL}`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
+  spec.files = Dir.chdir(__dir__) do
+    `git ls-files -z 2>#{IO::NULL}`.split("\x0").reject do |f|
+      (File.expand_path(f) == __FILE__) ||
+        f.start_with?(*%w[bin/ test/ spec/ features/ .git Gemfile])
+    end
   end
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }