title: Composer require branch name
body: <p>For example I want to require:</p>
  
  <pre><code>{
    "repositories": [
      {
        "type": "git",
        "url": "https://github.com/google/google-api-php-client.git"
      }
    ],
  
    "require": {
      "google/apiclient": "v1-master"
    }
  }
  </code></pre>
  
  <p>In this example I try require <code>google/apiclient</code> on branch <code>v1-master</code>. I get error:</p>
  
  <pre><code>  [UnexpectedValueException]                                                        
    Could not parse version constraint v1-master: Invalid version string "v1-master"
  </code></pre>