Use Dir#[] instead of git command for gemspec

For example, it prevents errors such as:

```console
bundle install

[!] There was an error parsing `Gemfile`:
[!] There was an error while loading `rubocop-performance.gemspec`: No such file or directory - git. Bundler cannot continue.

#  from /root/project/rubocop-performance.gemspec:18
#  -------------------------------------------
#    s.email = 'rubocop@googlegroups.com'
>    s.files = `git ls-files -z config lib LICENSE.txt README.md`.split("\x0")
#    s.extra_rdoc_files = ['LICENSE.txt', 'README.md']
#  -------------------------------------------
. Bundler cannot continue.

#  from /root/project/Gemfile:7
#  -------------------------------------------
#
>  gemspec
#
#  -------------------------------------------

Exited with code exit status 14
```

https://app.circleci.com/pipelines/github/rubocop/rubocop-performance/792

rubocop-performance.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
   DESCRIPTION
 
   s.email = 'rubocop@googlegroups.com'
-  s.files = `git ls-files -z config lib LICENSE.txt README.md`.split("\x0")
+  s.files = Dir['LICENSE.txt', 'README.md', 'config/**/*', 'lib/**/*']
   s.extra_rdoc_files = ['LICENSE.txt', 'README.md']
   s.homepage = 'https://github.com/rubocop/rubocop-performance'
   s.licenses = ['MIT']