title: Coverage in tox for multiple python versions
body: <p>Here is a <a href="https://drive.google.com/folderview?id=0B0bHr4crS9cpaWlockpxcmJxelE&amp;usp=drive_web" rel="noreferrer">link</a> to a project and output that you can use to reproduce the problem I describe below.</p>
  
  <p>I'm using <strong>coverage</strong> with <strong>tox</strong> against multiple versions of python. My tox.ini file looks something like this:</p>
  
  <pre><code>[tox]
  envlist =
      py27
      py34
  
  [testenv]
  deps =
      coverage
  
  commands =
      coverage run --source=modules/ -m pytest
      coverage report -m
  </code></pre>
  
  <p>My problem is that coverage will run using only one version of python (in my case, py27), not both py27 and py34. This is a problem whenever I have code execution dependent on the python version, e.g.:</p>
  
  <pre><code>def add(a, b):
      import sys
      if sys.version.startswith('2.7'):
          print('2.7')
      if sys.version.startswith('3'):
          print('3')
      return a + b
  </code></pre>
  
  <p>Running coverage against the above code will incorrectly report that line 6 ("print('3')") is "Missing" for both py27 and py34. It should only be Missing for py34.</p>
  
  <p>I know why this is happening: coverage is installed on my base OS (which uses python2.7). Thus, when <strong>tox</strong> is run, it notices that coverage is already installed and inherits coverage from the base OS rather than installing it in the virtualenv it creates.</p>
  
  <p>This is fine and dandy for py27, but causes incorrect results in the coverage report for py34. I have a hacky, temporary work-around: I require a slightly earlier version of coverage (relative to the one installed on my base OS) so that tox will be forced to install a separate copy of coverage in the virtualenv. E.g.</p>
  
  <pre><code>[testenv]
  deps =
      coverage==4.0.2
      pytest==2.9.0
      py==1.4.30
  </code></pre>
  
  <p>I don't like this workaround, but it's the best I've found for now. Any suggestions on a way to force tox to install the current version of coverage in its virtualenv's, even when I already have it installed on my base OS?</p>