title: Composer Github private repository not working with given key
body: <p>I was trying to get a private repository from Github installed via <code>composer</code>. My <code>composer</code> configuration is as below:</p>
  
  <pre><code>"repositories": [
              {
                  "type": "vcs",
                  "url":  "git@github.com:{user}/{repo}.git",
                  "options": {
                      "ssh2": {
                          "user": "ranacseruet" 
                          "privkey_file": "./keys/id_rsa",
                          "pubkey_file": "./keys/id_rsa.pub"
                      }
                  }
              }
          ]
  </code></pre>
  
  <p>However, it doesn't work with this configuration. It's still asking for github user/pass. However, if my system's github authorization is set up properly with keys, only then it works automatically. </p>
  
  <p>In any way, composer configuration isn't working. Is anybody else facing the same issue or is there anything I did wrong/need to check here? Thanks in advance.</p>