title: Install older (but stable) NLTK version compatible with python 2
body: <p>I would like to install an older (but stable) version of NLTK for python2.7.
  I tried to run the command: <code>pip install nltk===x.x.x</code> but the terminal reports many errors.
  I was wondering if there's a repository where nltk can be downloaded or whether there are some other ways to solve the problem. 
  Thanks  </p>