title: pip uses incorrect cached package version, instead of the user-specified version
body: <p>I need to install psycopg2 v2.4.1 specifically. I accidentally did:</p>
  
  <pre><code> pip install psycopg2
  </code></pre>
  
  <p>Instead of:</p>
  
  <pre><code> pip install psycopg2==2.4.1
  </code></pre>
  
  <p>That installs 2.4.4 instead of the earlier version. </p>
  
  <p>Now even after I pip uninstall psycopg2 and attempt to reinstall with the correct version, it appears that pip is re-using the cache it downloaded the first time.</p>
  
  <p>How can I force pip to clear out its download cache and use the specific version I'm including in the command?</p>