title: Meaning of new block "git_source(:github)" in Gemfile
body: <p>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:</p>
  
  <pre><code>git_source(:github) do |repo_name|
    repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
    "https://github.com/#{repo_name}.git"
  end
  </code></pre>
  
  <p>What's the meaning of it? Is it mandatory for every new app?</p>