title: Tox installs the wrong version of pip to it's virtual env
body: <p>I am using <a href="https://pypi.python.org/pypi/tox" rel="noreferrer">tox</a> to manage some testing environments. I have a dependency (backports.ssl-match-hostname) that I cannot download using the latest version of pip, so I need to revert back to pip 8.0.3 to allow the install to work.</p>
  
  <p>I have included the 8.0.3 version of pip inside my tox.ini file for dependencies.</p>
  
  <pre><code>deps=
      pip==8.0.3
  </code></pre>
  
  <p>However, when I run</p>
  
  <pre><code>source .tox/py27/bin/activate
  </code></pre>
  
  <p>and enter the virtual testing environment, and then run</p>
  
  <pre><code>pip --version
  </code></pre>
  
  <p>I end up with </p>
  
  <pre><code>8.1.2
  </code></pre>
  
  <p>However, outside of my tox environment, when I run the same command, I get</p>
  
  <pre><code>8.0.3
  </code></pre>
  
  <p>Is there anything special that tox does when grabbing pip? Why am I not able to specify the version of pip that I want to use as a dependency?</p>
  
  <p>EDIT : to add to this, it seems as though I am able to grab the dependency pip==8.0.3, but for the other dependencies, they are still running from the command launched with pip==8.1.2</p>
  
  <p>So, I need to be able to grab pip==8.0.3 first, and then once installed, grab everything else. Still unsure why tox is starting with pip==8.1.2</p>