title: How to reinstall requirements.txt
body: <p>I'm working in this shared Django project, a colleague is the owner of the repo in Github. The problem I am facing right now is that he added <strong>raven</strong> to his packages and in github the requirements.txt file is updated, however when I tried with git pull, locally, my requirements.txt does not have <strong>raven</strong> added. He told me that I have to reinstall requirements.txt so I tried with pip freeze > requirements.txt but nothing change.</p>
  
  <p>How can I update my requirements.txt file according the updates made from Github?</p>