title: can't install virtualenv on ubuntu 12.04
body: <p>i have to install virtualenv on ubuntu 12.04 but it shows following error</p>
  
  <pre><code>sudo pip install virtualenv
  [sudo] password for raushan: 
  Downloading/unpacking virtualenv
    Cannot fetch index base URL http://pypi.python.org/simple/
    Could not find any downloads that satisfy the requirement virtualenv
  No distributions at all found for virtualenv
  Storing complete log in /home/raushan/.pip/pip.log
  </code></pre>