title: Tox running fine in one repo clone but not the other
body: <p>I tried to run <code>tox</code> in a repo clone I have, but got this error:</p>
  
  <pre><code>helton@helton-pc /data/Dropbox/Forks do GitHub/fastkml $ sudo tox
  GLOB sdist-make: /data/Dropbox/Forks do GitHub/fastkml/setup.py
  py27 create: /data/Dropbox/Forks do GitHub/fastkml/.tox/py27
  py27 installdeps: pytest, coverage
  ERROR: invocation failed (errno 2), args: [local('/data/Dropbox/Forks do GitHub/fastkml/.tox/py27/bin/pip'), 'install', 'pytest', 'coverage'], cwd: /data/Dropbox/Forks do GitHub/fastkml
  Traceback (most recent call last):
    File "/usr/local/bin/tox", line 11, in &lt;module&gt;
      sys.exit(cmdline())
    File "/usr/local/lib/python2.7/dist-packages/tox/session.py", line 39, in main
      retcode = Session(config).runcommand()
    File "/usr/local/lib/python2.7/dist-packages/tox/session.py", line 375, in runcommand
      return self.subcommand_test()
    File "/usr/local/lib/python2.7/dist-packages/tox/session.py", line 526, in subcommand_test
      if self.setupenv(venv):
    File "/usr/local/lib/python2.7/dist-packages/tox/session.py", line 434, in setupenv
      status = venv.update(action=action)
    File "/usr/local/lib/python2.7/dist-packages/tox/venv.py", line 153, in update
      self.hook.tox_testenv_install_deps(action=action, venv=self)
    File "/usr/local/lib/python2.7/dist-packages/pluggy.py", line 724, in __call__
      return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
    File "/usr/local/lib/python2.7/dist-packages/pluggy.py", line 338, in _hookexec
      return self._inner_hookexec(hook, methods, kwargs)
    File "/usr/local/lib/python2.7/dist-packages/pluggy.py", line 333, in &lt;lambda&gt;
      _MultiCall(methods, kwargs, hook.spec_opts).execute()
    File "/usr/local/lib/python2.7/dist-packages/pluggy.py", line 596, in execute
      res = hook_impl.function(*args)
    File "/usr/local/lib/python2.7/dist-packages/tox/venv.py", line 401, in tox_testenv_install_deps
      venv._install(deps, action=action)
    File "/usr/local/lib/python2.7/dist-packages/tox/venv.py", line 284, in _install
      action=action)
    File "/usr/local/lib/python2.7/dist-packages/tox/venv.py", line 257, in run_install_command
      self._pcall(argv, cwd=self.envconfig.config.toxinidir, action=action)
    File "/usr/local/lib/python2.7/dist-packages/tox/venv.py", line 362, in _pcall
      redirect=redirect, ignore_ret=ignore_ret)
    File "/usr/local/lib/python2.7/dist-packages/tox/session.py", line 136, in popen
      stdout=stdout, stderr=STDOUT)
    File "/usr/local/lib/python2.7/dist-packages/tox/session.py", line 224, in _popen
      stdout=stdout, stderr=stderr, env=env)
    File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
      errread, errwrite)
    File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
      raise child_exception
  OSError: [Errno 2] No such file or directory
  </code></pre>
  
  <p>But when I cloned the repo to other location, it worked:</p>
  
  <pre><code>&gt; cd ~/Desktop
  &gt; git clone http://github.com/heltonbiker/fastkml
  &gt; cd fastkml
  &gt; tox
  </code></pre>
  
  <p>I am using Linux Mint 17.3 64bits with included python 2.7.6 and tox installed with <code>sudo pip install tox</code>.</p>