title: virtualenv is not activated when I run "pip install -r requirements.txt"
body: <p>I am running a ubuntu 10.04 virtual machine on AWS ec2. </p>
  
  <p>below is what happened to me:</p>
  
  <pre><code>source bin/activate
  (env)$ pip install -r requirements.txt
  (env)$ yolk -l  # no package was installed in my env
  deactivate
  $ yolk -l  # all packages were installed into /usr/lib and /usr/local
  
  source bin/activate
  (env)$ pip install simplejson==2.3.0
  (env)$ yolk -l  # simplejson was installed in my env
  </code></pre>
  
  <p>a side question is how can I remove all packages not in my env. </p>