title: npm install fails with Permission denied (publickey). Could not read from remote repository
body: <p>I have configured SSH key and added it to my GitHub account. 
  Then I have cloned private repo with ssh, and tried to install node modules (some of them are private): </p>
  
  <pre><code>    "package1": "git+ssh://git@github.com:company/package1.git#0.0.2",
      "package2": "git+ssh://git@github.com:company/package2.git#1.1.1",
  </code></pre>
  
  <p>and it fails with </p>
  
  <pre><code>npm ERR! git@github.com: Permission denied (publickey).
  npm ERR! fatal: Could not read from remote repository.
  </code></pre>