Recently I created a new Rails 5 app, without a git repository. The auto-generated Gemfile contains a new block I had not seen before:

```
git_source(:github) do |repo_name|
  repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
  "https://github.com/#{repo_name}.git"
end

```

What's the meaning of it? Is it mandatory for every new app?