title: python make virtualenv with specific python version fails
body: <p>I am used to ruby where you specify the ruby gemset and the ruby version using</p>
  
  <pre><code>.ruby-version
  .ruby-gemset
  </code></pre>
  
  <p>in python, it seems, that you have to use this syntax: </p>
  
  <pre><code>mkvirtualenv --python /usr/local/opt/pyenv/versions/3.4.2 test1
  </code></pre>
  
  <p>but it fails with this output:</p>
  
  <pre><code>Running virtualenv with interpreter /usr/local/opt/pyenv/versions/3.4.2
  Traceback (most recent call last):
    File "/usr/local/bin/virtualenv", line 11, in &lt;module&gt;
      sys.exit(main())
    File "/usr/local/lib/python2.7/site-packages/virtualenv.py", line 784, in main
      popen = subprocess.Popen([interpreter, file] + sys.argv[1:], env=env)
    File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 710, in __init__
      errread, errwrite)
    File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1335, in _execute_child
      raise child_exception
  OSError: [Errno 13] Permission denied
  </code></pre>
  
  <p>when running without specifying the python version it works like so:</p>
  
  <pre><code>mkvirtualenv test2                                                              ✱
  New python executable in test2/bin/python2.7
  Also creating executable in test2/bin/python
  Installing setuptools, pip...done.
  </code></pre>